diff options
Diffstat (limited to 'Documentation')
49 files changed, 10079 insertions, 3615 deletions
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+ <!-- Arrowhead on XXXpoint 5250 5850 - 5250 4740--> + </g> +</svg> diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.html b/Documentation/RCU/Design/Data-Structures/Data-Structures.html new file mode 100644 index 000000000000..7eb47ac25ad7 --- /dev/null +++ b/Documentation/RCU/Design/Data-Structures/Data-Structures.html @@ -0,0 +1,1333 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" + "http://www.w3.org/TR/html4/loose.dtd"> + <html> + <head><title>A Tour Through TREE_RCU's Data Structures [LWN.net]</title> + <meta HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1"> + + <p>January 27, 2016</p> + <p>This article was contributed by Paul E. McKenney</p> + +<h3>Introduction</h3> + +This document describes RCU's major data structures and their relationship +to each other. + +<ol> +<li> <a href="#Data-Structure Relationships"> + Data-Structure Relationships</a> +<li> <a href="#The rcu_state Structure"> + The <tt>rcu_state</tt> Structure</a> +<li> <a href="#The rcu_node Structure"> + The <tt>rcu_node</tt> Structure</a> +<li> <a href="#The rcu_data Structure"> + The <tt>rcu_data</tt> Structure</a> +<li> <a href="#The rcu_dynticks Structure"> + The <tt>rcu_dynticks</tt> Structure</a> +<li> <a href="#The rcu_head Structure"> + The <tt>rcu_head</tt> Structure</a> +<li> <a href="#RCU-Specific Fields in the task_struct Structure"> + RCU-Specific Fields in the <tt>task_struct</tt> Structure</a> +<li> <a href="#Accessor Functions"> + Accessor Functions</a> +</ol> + +At the end we have the +<a href="#Answers to Quick Quizzes">answers to the quick quizzes</a>. + +<h3><a name="Data-Structure Relationships">Data-Structure Relationships</a></h3> + +<p>RCU is for all intents and purposes a large state machine, and its +data structures maintain the state in such a way as to allow RCU readers +to execute extremely quickly, while also processing the RCU grace periods +requested by updaters in an efficient and extremely scalable fashion. +The efficiency and scalability of RCU updaters is provided primarily +by a combining tree, as shown below: + +</p><p><img src="BigTreeClassicRCU.svg" alt="BigTreeClassicRCU.svg" width="30%"> + +</p><p>This diagram shows an enclosing <tt>rcu_state</tt> structure +containing a tree of <tt>rcu_node</tt> structures. +Each leaf node of the <tt>rcu_node</tt> tree has up to 16 +<tt>rcu_data</tt> structures associated with it, so that there +are <tt>NR_CPUS</tt> number of <tt>rcu_data</tt> structures, +one for each possible CPU. +This structure is adjusted at boot time, if needed, to handle the +common case where <tt>nr_cpu_ids</tt> is much less than +<tt>NR_CPUs</tt>. +For example, a number of Linux distributions set <tt>NR_CPUs=4096</tt>, +which results in a three-level <tt>rcu_node</tt> tree. +If the actual hardware has only 16 CPUs, RCU will adjust itself +at boot time, resulting in an <tt>rcu_node</tt> tree with only a single node. + +</p><p>The purpose of this combining tree is to allow per-CPU events +such as quiescent states, dyntick-idle transitions, +and CPU hotplug operations to be processed efficiently +and scalably. +Quiescent states are recorded by the per-CPU <tt>rcu_data</tt> structures, +and other events are recorded by the leaf-level <tt>rcu_node</tt> +structures. +All of these events are combined at each level of the tree until finally +grace periods are completed at the tree's root <tt>rcu_node</tt> +structure. +A grace period can be completed at the root once every CPU +(or, in the case of <tt>CONFIG_PREEMPT_RCU</tt>, task) +has passed through a quiescent state. +Once a grace period has completed, record of that fact is propagated +back down the tree. + +</p><p>As can be seen from the diagram, on a 64-bit system +a two-level tree with 64 leaves can accommodate 1,024 CPUs, with a fanout +of 64 at the root and a fanout of 16 at the leaves. + +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + Why isn't the fanout at the leaves also 64? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + Because there are more types of events that affect the leaf-level + <tt>rcu_node</tt> structures than further up the tree. + Therefore, if the leaf <tt>rcu_node</tt> structures have fanout of + 64, the contention on these structures' <tt>->structures</tt> + becomes excessive. + Experimentation on a wide variety of systems has shown that a fanout + of 16 works well for the leaves of the <tt>rcu_node</tt> tree. + </font> + + <p><font color="ffffff">Of course, further experience with + systems having hundreds or thousands of CPUs may demonstrate + that the fanout for the non-leaf <tt>rcu_node</tt> structures + must also be reduced. + Such reduction can be easily carried out when and if it proves + necessary. + In the meantime, if you are using such a system and running into + contention problems on the non-leaf <tt>rcu_node</tt> structures, + you may use the <tt>CONFIG_RCU_FANOUT</tt> kernel configuration + parameter to reduce the non-leaf fanout as needed. + </font> + + <p><font color="ffffff">Kernels built for systems with + strong NUMA characteristics might also need to adjust + <tt>CONFIG_RCU_FANOUT</tt> so that the domains of the + <tt>rcu_node</tt> structures align with hardware boundaries. + However, there has thus far been no need for this. +</font></td></tr> +<tr><td> </td></tr> +</table> + +<p>If your system has more than 1,024 CPUs (or more than 512 CPUs on +a 32-bit system), then RCU will automatically add more levels to the +tree. +For example, if you are crazy enough to build a 64-bit system with 65,536 +CPUs, RCU would configure the <tt>rcu_node</tt> tree as follows: + +</p><p><img src="HugeTreeClassicRCU.svg" alt="HugeTreeClassicRCU.svg" width="50%"> + +</p><p>RCU currently permits up to a four-level tree, which on a 64-bit system +accommodates up to 4,194,304 CPUs, though only a mere 524,288 CPUs for +32-bit systems. +On the other hand, you can set <tt>CONFIG_RCU_FANOUT</tt> to be +as small as 2 if you wish, which would permit only 16 CPUs, which +is useful for testing. + +</p><p>This multi-level combining tree allows us to get most of the +performance and scalability +benefits of partitioning, even though RCU grace-period detection is +inherently a global operation. +The trick here is that only the last CPU to report a quiescent state +into a given <tt>rcu_node</tt> structure need advance to the <tt>rcu_node</tt> +structure at the next level up the tree. +This means that at the leaf-level <tt>rcu_node</tt> structure, only +one access out of sixteen will progress up the tree. +For the internal <tt>rcu_node</tt> structures, the situation is even +more extreme: Only one access out of sixty-four will progress up +the tree. +Because the vast majority of the CPUs do not progress up the tree, +the lock contention remains roughly constant up the tree. +No matter how many CPUs there are in the system, at most 64 quiescent-state +reports per grace period will progress all the way to the root +<tt>rcu_node</tt> structure, thus ensuring that the lock contention +on that root <tt>rcu_node</tt> structure remains acceptably low. + +</p><p>In effect, the combining tree acts like a big shock absorber, +keeping lock contention under control at all tree levels regardless +of the level of loading on the system. + +</p><p>The Linux kernel actually supports multiple flavors of RCU +running concurrently, so RCU builds separate data structures for each +flavor. +For example, for <tt>CONFIG_TREE_RCU=y</tt> kernels, RCU provides +rcu_sched and rcu_bh, as shown below: + +</p><p><img src="BigTreeClassicRCUBH.svg" alt="BigTreeClassicRCUBH.svg" width="33%"> + +</p><p>Energy efficiency is increasingly important, and for that +reason the Linux kernel provides <tt>CONFIG_NO_HZ_IDLE</tt>, which +turns off the scheduling-clock interrupts on idle CPUs, which in +turn allows those CPUs to attain deeper sleep states and to consume +less energy. +CPUs whose scheduling-clock interrupts have been turned off are +said to be in <i>dyntick-idle mode</i>. +RCU must handle dyntick-idle CPUs specially +because RCU would otherwise wake up each CPU on every grace period, +which would defeat the whole purpose of <tt>CONFIG_NO_HZ_IDLE</tt>. +RCU uses the <tt>rcu_dynticks</tt> structure to track +which CPUs are in dyntick idle mode, as shown below: + +</p><p><img src="BigTreeClassicRCUBHdyntick.svg" alt="BigTreeClassicRCUBHdyntick.svg" width="33%"> + +</p><p>However, if a CPU is in dyntick-idle mode, it is in that mode +for all flavors of RCU. +Therefore, a single <tt>rcu_dynticks</tt> structure is allocated per +CPU, and all of a given CPU's <tt>rcu_data</tt> structures share +that <tt>rcu_dynticks</tt>, as shown in the figure. + +</p><p>Kernels built with <tt>CONFIG_PREEMPT_RCU</tt> support +rcu_preempt in addition to rcu_sched and rcu_bh, as shown below: + +</p><p><img src="BigTreePreemptRCUBHdyntick.svg" alt="BigTreePreemptRCUBHdyntick.svg" width="35%"> + +</p><p>RCU updaters wait for normal grace periods by registering +RCU callbacks, either directly via <tt>call_rcu()</tt> and +friends (namely <tt>call_rcu_bh()</tt> and <tt>call_rcu_sched()</tt>), +there being a separate interface per flavor of RCU) +or indirectly via <tt>synchronize_rcu()</tt> and friends. +RCU callbacks are represented by <tt>rcu_head</tt> structures, +which are queued on <tt>rcu_data</tt> structures while they are +waiting for a grace period to elapse, as shown in the following figure: + +</p><p><img src="BigTreePreemptRCUBHdyntickCB.svg" alt="BigTreePreemptRCUBHdyntickCB.svg" width="40%"> + +</p><p>This figure shows how <tt>TREE_RCU</tt>'s and +<tt>PREEMPT_RCU</tt>'s major data structures are related. +Lesser data structures will be introduced with the algorithms that +make use of them. + +</p><p>Note that each of the data structures in the above figure has +its own synchronization: + +<p><ol> +<li> Each <tt>rcu_state</tt> structures has a lock and a mutex, + and some fields are protected by the corresponding root + <tt>rcu_node</tt> structure's lock. +<li> Each <tt>rcu_node</tt> structure has a spinlock. +<li> The fields in <tt>rcu_data</tt> are private to the corresponding + CPU, although a few can be read and written by other CPUs. +<li> Similarly, the fields in <tt>rcu_dynticks</tt> are private + to the corresponding CPU, although a few can be read by + other CPUs. +</ol> + +<p>It is important to note that different data structures can have +very different ideas about the state of RCU at any given time. +For but one example, awareness of the start or end of a given RCU +grace period propagates slowly through the data structures. +This slow propagation is absolutely necessary for RCU to have good +read-side performance. +If this balkanized implementation seems foreign to you, one useful +trick is to consider each instance of these data structures to be +a different person, each having the usual slightly different +view of reality. + +</p><p>The general role of each of these data structures is as +follows: + +</p><ol> +<li> <tt>rcu_state</tt>: + This structure forms the interconnection between the + <tt>rcu_node</tt> and <tt>rcu_data</tt> structures, + tracks grace periods, serves as short-term repository + for callbacks orphaned by CPU-hotplug events, + maintains <tt>rcu_barrier()</tt> state, + tracks expedited grace-period state, + and maintains state used to force quiescent states when + grace periods extend too long, +<li> <tt>rcu_node</tt>: This structure forms the combining + tree that propagates quiescent-state + information from the leaves to the root, and also propagates + grace-period information from the root to the leaves. + It provides local copies of the grace-period state in order + to allow this information to be accessed in a synchronized + manner without suffering the scalability limitations that + would otherwise be imposed by global locking. + In <tt>CONFIG_PREEMPT_RCU</tt> kernels, it manages the lists + of tasks that have blocked while in their current + RCU read-side critical section. + In <tt>CONFIG_PREEMPT_RCU</tt> with + <tt>CONFIG_RCU_BOOST</tt>, it manages the + per-<tt>rcu_node</tt> priority-boosting + kernel threads (kthreads) and state. + Finally, it records CPU-hotplug state in order to determine + which CPUs should be ignored during a given grace period. +<li> <tt>rcu_data</tt>: This per-CPU structure is the + focus of quiescent-state detection and RCU callback queuing. + It also tracks its relationship to the corresponding leaf + <tt>rcu_node</tt> structure to allow more-efficient + propagation of quiescent states up the <tt>rcu_node</tt> + combining tree. + Like the <tt>rcu_node</tt> structure, it provides a local + copy of the grace-period information to allow for-free + synchronized + access to this information from the corresponding CPU. + Finally, this structure records past dyntick-idle state + for the corresponding CPU and also tracks statistics. +<li> <tt>rcu_dynticks</tt>: + This per-CPU structure tracks the current dyntick-idle + state for the corresponding CPU. + Unlike the other three structures, the <tt>rcu_dynticks</tt> + structure is not replicated per RCU flavor. +<li> <tt>rcu_head</tt>: + This structure represents RCU callbacks, and is the + only structure allocated and managed by RCU users. + The <tt>rcu_head</tt> structure is normally embedded + within the RCU-protected data structure. +</ol> + +<p>If all you wanted from this article was a general notion of how +RCU's data structures are related, you are done. +Otherwise, each of the following sections give more details on +the <tt>rcu_state</tt>, <tt>rcu_node</tt>, <tt>rcu_data</tt>, +and <tt>rcu_dynticks</tt> data structures. + +<h3><a name="The rcu_state Structure"> +The <tt>rcu_state</tt> Structure</a></h3> + +<p>The <tt>rcu_state</tt> structure is the base structure that +represents a flavor of RCU. +This structure forms the interconnection between the +<tt>rcu_node</tt> and <tt>rcu_data</tt> structures, +tracks grace periods, contains the lock used to +synchronize with CPU-hotplug events, +and maintains state used to force quiescent states when +grace periods extend too long, + +</p><p>A few of the <tt>rcu_state</tt> structure's fields are discussed, +singly and in groups, in the following sections. +The more specialized fields are covered in the discussion of their +use. + +<h5>Relationship to rcu_node and rcu_data Structures</h5> + +This portion of the <tt>rcu_state</tt> structure is declared +as follows: + +<pre> + 1 struct rcu_node node[NUM_RCU_NODES]; + 2 struct rcu_node *level[NUM_RCU_LVLS + 1]; + 3 struct rcu_data __percpu *rda; +</pre> + +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + Wait a minute! + You said that the <tt>rcu_node</tt> structures formed a tree, + but they are declared as a flat array! + What gives? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + The tree is laid out in the array. + The first node In the array is the head, the next set of nodes in the + array are children of the head node, and so on until the last set of + nodes in the array are the leaves. + </font> + + <p><font color="ffffff">See the following diagrams to see how + this works. +</font></td></tr> +<tr><td> </td></tr> +</table> + +<p>The <tt>rcu_node</tt> tree is embedded into the +<tt>->node[]</tt> array as shown in the following figure: + +</p><p><img src="TreeMapping.svg" alt="TreeMapping.svg" width="40%"> + +</p><p>One interesting consequence of this mapping is that a +breadth-first traversal of the tree is implemented as a simple +linear scan of the array, which is in fact what the +<tt>rcu_for_each_node_breadth_first()</tt> macro does. +This macro is used at the beginning and ends of grace periods. + +</p><p>Each entry of the <tt>->level</tt> array references +the first <tt>rcu_node</tt> structure on the corresponding level +of the tree, for example, as shown below: + +</p><p><img src="TreeMappingLevel.svg" alt="TreeMappingLevel.svg" width="40%"> + +</p><p>The zero<sup>th</sup> element of the array references the root +<tt>rcu_node</tt> structure, the first element references the +first child of the root <tt>rcu_node</tt>, and finally the second +element references the first leaf <tt>rcu_node</tt> structure. + +</p><p>For whatever it is worth, if you draw the tree to be tree-shaped +rather than array-shaped, it is easy to draw a planar representation: + +</p><p><img src="TreeLevel.svg" alt="TreeLevel.svg" width="60%"> + +</p><p>Finally, the <tt>->rda</tt> field references a per-CPU +pointer to the corresponding CPU's <tt>rcu_data</tt> structure. + +</p><p>All of these fields are constant once initialization is complete, +and therefore need no protection. + +<h5>Grace-Period Tracking</h5> + +<p>This portion of the <tt>rcu_state</tt> structure is declared +as follows: + +<pre> + 1 unsigned long gpnum; + 2 unsigned long completed; +</pre> + +<p>RCU grace periods are numbered, and +the <tt>->gpnum</tt> field contains the number of the grace +period that started most recently. +The <tt>->completed</tt> field contains the number of the +grace period that completed most recently. +If the two fields are equal, the RCU grace period that most recently +started has already completed, and therefore the corresponding +flavor of RCU is idle. +If <tt>->gpnum</tt> is one greater than <tt>->completed</tt>, +then <tt>->gpnum</tt> gives the number of the current RCU +grace period, which has not yet completed. +Any other combination of values indicates that something is broken. +These two fields are protected by the root <tt>rcu_node</tt>'s +<tt>->lock</tt> field. + +</p><p>There are <tt>->gpnum</tt> and <tt>->completed</tt> fields +in the <tt>rcu_node</tt> and <tt>rcu_data</tt> structures +as well. +The fields in the <tt>rcu_state</tt> structure represent the +most current values, and those of the other structures are compared +in order to detect the start of a new grace period in a distributed +fashion. +The values flow from <tt>rcu_state</tt> to <tt>rcu_node</tt> +(down the tree from the root to the leaves) to <tt>rcu_data</tt>. + +<h5>Miscellaneous</h5> + +<p>This portion of the <tt>rcu_state</tt> structure is declared +as follows: + +<pre> + 1 unsigned long gp_max; + 2 char abbr; + 3 char *name; +</pre> + +<p>The <tt>->gp_max</tt> field tracks the duration of the longest +grace period in jiffies. +It is protected by the root <tt>rcu_node</tt>'s <tt>->lock</tt>. + +<p>The <tt>->name</tt> field points to the name of the RCU flavor +(for example, “rcu_sched”), and is constant. +The <tt>->abbr</tt> field contains a one-character abbreviation, +for example, “s” for RCU-sched. + +<h3><a name="The rcu_node Structure"> +The <tt>rcu_node</tt> Structure</a></h3> + +<p>The <tt>rcu_node</tt> structures form the combining +tree that propagates quiescent-state +information from the leaves to the root and also that propagates +grace-period information from the root down to the leaves. +They provides local copies of the grace-period state in order +to allow this information to be accessed in a synchronized +manner without suffering the scalability limitations that +would otherwise be imposed by global locking. +In <tt>CONFIG_PREEMPT_RCU</tt> kernels, they manage the lists +of tasks that have blocked while in their current +RCU read-side critical section. +In <tt>CONFIG_PREEMPT_RCU</tt> with +<tt>CONFIG_RCU_BOOST</tt>, they manage the +per-<tt>rcu_node</tt> priority-boosting +kernel threads (kthreads) and state. +Finally, they record CPU-hotplug state in order to determine +which CPUs should be ignored during a given grace period. + +</p><p>The <tt>rcu_node</tt> structure's fields are discussed, +singly and in groups, in the following sections. + +<h5>Connection to Combining Tree</h5> + +<p>This portion of the <tt>rcu_node</tt> structure is declared +as follows: + +<pre> + 1 struct rcu_node *parent; + 2 u8 level; + 3 u8 grpnum; + 4 unsigned long grpmask; + 5 int grplo; + 6 int grphi; +</pre> + +<p>The <tt>->parent</tt> pointer references the <tt>rcu_node</tt> +one level up in the tree, and is <tt>NULL</tt> for the root +<tt>rcu_node</tt>. +The RCU implementation makes heavy use of this field to push quiescent +states up the tree. +The <tt>->level</tt> field gives the level in the tree, with +the root being at level zero, its children at level one, and so on. +The <tt>->grpnum</tt> field gives this node's position within +the children of its parent, so this number can range between 0 and 31 +on 32-bit systems and between 0 and 63 on 64-bit systems. +The <tt>->level</tt> and <tt>->grpnum</tt> fields are +used only during initialization and for tracing. +The <tt>->grpmask</tt> field is the bitmask counterpart of +<tt>->grpnum</tt>, and therefore always has exactly one bit set. +This mask is used to clear the bit corresponding to this <tt>rcu_node</tt> +structure in its parent's bitmasks, which are described later. +Finally, the <tt>->grplo</tt> and <tt>->grphi</tt> fields +contain the lowest and highest numbered CPU served by this +<tt>rcu_node</tt> structure, respectively. + +</p><p>All of these fields are constant, and thus do not require any +synchronization. + +<h5>Synchronization</h5> + +<p>This field of the <tt>rcu_node</tt> structure is declared +as follows: + +<pre> + 1 raw_spinlock_t lock; +</pre> + +<p>This field is used to protect the remaining fields in this structure, +unless otherwise stated. +That said, all of the fields in this structure can be accessed without +locking for tracing purposes. +Yes, this can result in confusing traces, but better some tracing confusion +than to be heisenbugged out of existence. + +<h5>Grace-Period Tracking</h5> + +<p>This portion of the <tt>rcu_node</tt> structure is declared +as follows: + +<pre> + 1 unsigned long gpnum; + 2 unsigned long completed; +</pre> + +<p>These fields are the counterparts of the fields of the same name in +the <tt>rcu_state</tt> structure. +They each may lag up to one behind their <tt>rcu_state</tt> +counterparts. +If a given <tt>rcu_node</tt> structure's <tt>->gpnum</tt> and +<tt>->complete</tt> fields are equal, then this <tt>rcu_node</tt> +structure believes that RCU is idle. +Otherwise, as with the <tt>rcu_state</tt> structure, +the <tt>->gpnum</tt> field will be one greater than the +<tt>->complete</tt> fields, with <tt>->gpnum</tt> +indicating which grace period this <tt>rcu_node</tt> believes +is still being waited for. + +</p><p>The <tt>>gpnum</tt> field of each <tt>rcu_node</tt> +structure is updated at the beginning +of each grace period, and the <tt>->completed</tt> fields are +updated at the end of each grace period. + +<h5>Quiescent-State Tracking</h5> + +<p>These fields manage the propagation of quiescent states up the +combining tree. + +</p><p>This portion of the <tt>rcu_node</tt> structure has fields +as follows: + +<pre> + 1 unsigned long qsmask; + 2 unsigned long expmask; + 3 unsigned long qsmaskinit; + 4 unsigned long expmaskinit; +</pre> + +<p>The <tt>->qsmask</tt> field tracks which of this +<tt>rcu_node</tt> structure's children still need to report +quiescent states for the current normal grace period. +Such children will have a value of 1 in their corresponding bit. +Note that the leaf <tt>rcu_node</tt> structures should be +thought of as having <tt>rcu_data</tt> structures as their +children. +Similarly, the <tt>->expmask</tt> field tracks which +of this <tt>rcu_node</tt> structure's children still need to report +quiescent states for the current expedited grace period. +An expedited grace period has +the same conceptual properties as a normal grace period, but the +expedited implementation accepts extreme CPU overhead to obtain +much lower grace-period latency, for example, consuming a few +tens of microseconds worth of CPU time to reduce grace-period +duration from milliseconds to tens of microseconds. +The <tt>->qsmaskinit</tt> field tracks which of this +<tt>rcu_node</tt> structure's children cover for at least +one online CPU. +This mask is used to initialize <tt>->qsmask</tt>, +and <tt>->expmaskinit</tt> is used to initialize +<tt>->expmask</tt> and the beginning of the +normal and expedited grace periods, respectively. + +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + Why are these bitmasks protected by locking? + Come on, haven't you heard of atomic instructions??? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + Lockless grace-period computation! Such a tantalizing possibility! + </font> + + <p><font color="ffffff">But consider the following sequence of events: + </font> + + <ol> + <li> <font color="ffffff">CPU 0 has been in dyntick-idle + mode for quite some time. + When it wakes up, it notices that the current RCU + grace period needs it to report in, so it sets a + flag where the scheduling clock interrupt will find it. + </font><p> + <li> <font color="ffffff">Meanwhile, CPU 1 is running + <tt>force_quiescent_state()</tt>, + and notices that CPU 0 has been in dyntick idle mode, + which qualifies as an extended quiescent state. + </font><p> + <li> <font color="ffffff">CPU 0's scheduling clock + interrupt fires in the + middle of an RCU read-side critical section, and notices + that the RCU core needs something, so commences RCU softirq + processing. + </font> + <p> + <li> <font color="ffffff">CPU 0's softirq handler + executes and is just about ready + to report its quiescent state up the <tt>rcu_node</tt> + tree. + </font><p> + <li> <font color="ffffff">But CPU 1 beats it to the punch, + completing the current + grace period and starting a new one. + </font><p> + <li> <font color="ffffff">CPU 0 now reports its quiescent + state for the wrong + grace period. + That grace period might now end before the RCU read-side + critical section. + If that happens, disaster will ensue. + </font> + </ol> + + <p><font color="ffffff">So the locking is absolutely required in + order to coordinate + clearing of the bits with the grace-period numbers in + <tt>->gpnum</tt> and <tt>->completed</tt>. +</font></td></tr> +<tr><td> </td></tr> +</table> + +<h5>Blocked-Task Management</h5> + +<p><tt>PREEMPT_RCU</tt> allows tasks to be preempted in the +midst of their RCU read-side critical sections, and these tasks +must be tracked explicitly. +The details of exactly why and how they are tracked will be covered +in a separate article on RCU read-side processing. +For now, it is enough to know that the <tt>rcu_node</tt> +structure tracks them. + +<pre> + 1 struct list_head blkd_tasks; + 2 struct list_head *gp_tasks; + 3 struct list_head *exp_tasks; + 4 bool wait_blkd_tasks; +</pre> + +<p>The <tt>->blkd_tasks</tt> field is a list header for +the list of blocked and preempted tasks. +As tasks undergo context switches within RCU read-side critical +sections, their <tt>task_struct</tt> structures are enqueued +(via the <tt>task_struct</tt>'s <tt>->rcu_node_entry</tt> +field) onto the head of the <tt>->blkd_tasks</tt> list for the +leaf <tt>rcu_node</tt> structure corresponding to the CPU +on which the outgoing context switch executed. +As these tasks later exit their RCU read-side critical sections, +they remove themselves from the list. +This list is therefore in reverse time order, so that if one of the tasks +is blocking the current grace period, all subsequent tasks must +also be blocking that same grace period. +Therefore, a single pointer into this list suffices to track +all tasks blocking a given grace period. +That pointer is stored in <tt>->gp_tasks</tt> for normal +grace periods and in <tt>->exp_tasks</tt> for expedited +grace periods. +These last two fields are <tt>NULL</tt> if either there is +no grace period in flight or if there are no blocked tasks +preventing that grace period from completing. +If either of these two pointers is referencing a task that +removes itself from the <tt>->blkd_tasks</tt> list, +then that task must advance the pointer to the next task on +the list, or set the pointer to <tt>NULL</tt> if there +are no subsequent tasks on the list. + +</p><p>For example, suppose that tasks T1, T2, and T3 are +all hard-affinitied to the largest-numbered CPU in the system. +Then if task T1 blocked in an RCU read-side +critical section, then an expedited grace period started, +then task T2 blocked in an RCU read-side critical section, +then a normal grace period started, and finally task 3 blocked +in an RCU read-side critical section, then the state of the +last leaf <tt>rcu_node</tt> structure's blocked-task list +would be as shown below: + +</p><p><img src="blkd_task.svg" alt="blkd_task.svg" width="60%"> + +</p><p>Task T1 is blocking both grace periods, task T2 is +blocking only the normal grace period, and task T3 is blocking +neither grace period. +Note that these tasks will not remove themselves from this list +immediately upon resuming execution. +They will instead remain on the list until they execute the outermost +<tt>rcu_read_unlock()</tt> that ends their RCU read-side critical +section. + +<p> +The <tt>->wait_blkd_tasks</tt> field indicates whether or not +the current grace period is waiting on a blocked task. + +<h5>Sizing the <tt>rcu_node</tt> Array</h5> + +<p>The <tt>rcu_node</tt> array is sized via a series of +C-preprocessor expressions as follows: + +<pre> + 1 #ifdef CONFIG_RCU_FANOUT + 2 #define RCU_FANOUT CONFIG_RCU_FANOUT + 3 #else + 4 # ifdef CONFIG_64BIT + 5 # define RCU_FANOUT 64 + 6 # else + 7 # define RCU_FANOUT 32 + 8 # endif + 9 #endif +10 +11 #ifdef CONFIG_RCU_FANOUT_LEAF +12 #define RCU_FANOUT_LEAF CONFIG_RCU_FANOUT_LEAF +13 #else +14 # ifdef CONFIG_64BIT +15 # define RCU_FANOUT_LEAF 64 +16 # else +17 # define RCU_FANOUT_LEAF 32 +18 # endif +19 #endif +20 +21 #define RCU_FANOUT_1 (RCU_FANOUT_LEAF) +22 #define RCU_FANOUT_2 (RCU_FANOUT_1 * RCU_FANOUT) +23 #define RCU_FANOUT_3 (RCU_FANOUT_2 * RCU_FANOUT) +24 #define RCU_FANOUT_4 (RCU_FANOUT_3 * RCU_FANOUT) +25 +26 #if NR_CPUS <= RCU_FANOUT_1 +27 # define RCU_NUM_LVLS 1 +28 # define NUM_RCU_LVL_0 1 +29 # define NUM_RCU_NODES NUM_RCU_LVL_0 +30 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0 } +31 # define RCU_NODE_NAME_INIT { "rcu_node_0" } +32 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0" } +33 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0" } +34 #elif NR_CPUS <= RCU_FANOUT_2 +35 # define RCU_NUM_LVLS 2 +36 # define NUM_RCU_LVL_0 1 +37 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1) +38 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1) +39 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1 } +40 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1" } +41 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1" } +42 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1" } +43 #elif NR_CPUS <= RCU_FANOUT_3 +44 # define RCU_NUM_LVLS 3 +45 # define NUM_RCU_LVL_0 1 +46 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2) +47 # define NUM_RCU_LVL_2 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1) +48 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2) +49 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2 } +50 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1", "rcu_node_2" } +51 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2" } +52 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2" } +53 #elif NR_CPUS <= RCU_FANOUT_4 +54 # define RCU_NUM_LVLS 4 +55 # define NUM_RCU_LVL_0 1 +56 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_3) +57 # define NUM_RCU_LVL_2 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2) +58 # define NUM_RCU_LVL_3 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1) +59 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2 + NUM_RCU_LVL_3) +60 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2, NUM_RCU_LVL_3 } +61 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1", "rcu_node_2", "rcu_node_3" } +62 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2", "rcu_node_fqs_3" } +63 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2", "rcu_node_exp_3" } +64 #else +65 # error "CONFIG_RCU_FANOUT insufficient for NR_CPUS" +66 #endif +</pre> + +<p>The maximum number of levels in the <tt>rcu_node</tt> structure +is currently limited to four, as specified by lines 21-24 +and the structure of the subsequent “if” statement. +For 32-bit systems, this allows 16*32*32*32=524,288 CPUs, which +should be sufficient for the next few years at least. +For 64-bit systems, 16*64*64*64=4,194,304 CPUs is allowed, which +should see us through the next decade or so. +This four-level tree also allows kernels built with +<tt>CONFIG_RCU_FANOUT=8</tt> to support up to 4096 CPUs, +which might be useful in very large systems having eight CPUs per +socket (but please note that no one has yet shown any measurable +performance degradation due to misaligned socket and <tt>rcu_node</tt> +boundaries). +In addition, building kernels with a full four levels of <tt>rcu_node</tt> +tree permits better testing of RCU's combining-tree code. + +</p><p>The <tt>RCU_FANOUT</tt> symbol controls how many children +are permitted at each non-leaf level of the <tt>rcu_node</tt> tree. +If the <tt>CONFIG_RCU_FANOUT</tt> Kconfig option is not specified, +it is set based on the word size of the system, which is also +the Kconfig default. + +</p><p>The <tt>RCU_FANOUT_LEAF</tt> symbol controls how many CPUs are +handled by each leaf <tt>rcu_node</tt> structure. +Experience has shown that allowing a given leaf <tt>rcu_node</tt> +structure to handle 64 CPUs, as permitted by the number of bits in +the <tt>->qsmask</tt> field on a 64-bit system, results in +excessive contention for the leaf <tt>rcu_node</tt> structures' +<tt>->lock</tt> fields. +The number of CPUs per leaf <tt>rcu_node</tt> structure is therefore +limited to 16 given the default value of <tt>CONFIG_RCU_FANOUT_LEAF</tt>. +If <tt>CONFIG_RCU_FANOUT_LEAF</tt> is unspecified, the value +selected is based on the word size of the system, just as for +<tt>CONFIG_RCU_FANOUT</tt>. +Lines 11-19 perform this computation. + +</p><p>Lines 21-24 compute the maximum number of CPUs supported by +a single-level (which contains a single <tt>rcu_node</tt> structure), +two-level, three-level, and four-level <tt>rcu_node</tt> tree, +respectively, given the fanout specified by <tt>RCU_FANOUT</tt> +and <tt>RCU_FANOUT_LEAF</tt>. +These numbers of CPUs are retained in the +<tt>RCU_FANOUT_1</tt>, +<tt>RCU_FANOUT_2</tt>, +<tt>RCU_FANOUT_3</tt>, and +<tt>RCU_FANOUT_4</tt> +C-preprocessor variables, respectively. + +</p><p>These variables are used to control the C-preprocessor <tt>#if</tt> +statement spanning lines 26-66 that computes the number of +<tt>rcu_node</tt> structures required for each level of the tree, +as well as the number of levels required. +The number of levels is placed in the <tt>NUM_RCU_LVLS</tt> +C-preprocessor variable by lines 27, 35, 44, and 54. +The number of <tt>rcu_node</tt> structures for the topmost level +of the tree is always exactly one, and this value is unconditionally +placed into <tt>NUM_RCU_LVL_0</tt> by lines 28, 36, 45, and 55. +The rest of the levels (if any) of the <tt>rcu_node</tt> tree +are computed by dividing the maximum number of CPUs by the +fanout supported by the number of levels from the current level down, +rounding up. This computation is performed by lines 37, +46-47, and 56-58. +Lines 31-33, 40-42, 50-52, and 62-63 create initializers +for lockdep lock-class names. +Finally, lines 64-66 produce an error if the maximum number of +CPUs is too large for the specified fanout. + +<h3><a name="The rcu_data Structure"> +The <tt>rcu_data</tt> Structure</a></h3> + +<p>The <tt>rcu_data</tt> maintains the per-CPU state for the +corresponding flavor of RCU. +The fields in this structure may be accessed only from the corresponding +CPU (and from tracing) unless otherwise stated. +This structure is the +focus of quiescent-state detection and RCU callback queuing. +It also tracks its relationship to the corresponding leaf +<tt>rcu_node</tt> structure to allow more-efficient +propagation of quiescent states up the <tt>rcu_node</tt> +combining tree. +Like the <tt>rcu_node</tt> structure, it provides a local +copy of the grace-period information to allow for-free +synchronized +access to this information from the corresponding CPU. +Finally, this structure records past dyntick-idle state +for the corresponding CPU and also tracks statistics. + +</p><p>The <tt>rcu_data</tt> structure's fields are discussed, +singly and in groups, in the following sections. + +<h5>Connection to Other Data Structures</h5> + +<p>This portion of the <tt>rcu_data</tt> structure is declared +as follows: + +<pre> + 1 int cpu; + 2 struct rcu_state *rsp; + 3 struct rcu_node *mynode; + 4 struct rcu_dynticks *dynticks; + 5 unsigned long grpmask; + 6 bool beenonline; +</pre> + +<p>The <tt>->cpu</tt> field contains the number of the +corresponding CPU, the <tt>->rsp</tt> pointer references +the corresponding <tt>rcu_state</tt> structure (and is most frequently +used to locate the name of the corresponding flavor of RCU for tracing), +and the <tt>->mynode</tt> field references the corresponding +<tt>rcu_node</tt> structure. +The <tt>->mynode</tt> is used to propagate quiescent states +up the combining tree. +<p>The <tt>->dynticks</tt> pointer references the +<tt>rcu_dynticks</tt> structure corresponding to this +CPU. +Recall that a single per-CPU instance of the <tt>rcu_dynticks</tt> +structure is shared among all flavors of RCU. +These first four fields are constant and therefore require not +synchronization. + +</p><p>The <tt>->grpmask</tt> field indicates the bit in +the <tt>->mynode->qsmask</tt> corresponding to this +<tt>rcu_data</tt> structure, and is also used when propagating +quiescent states. +The <tt>->beenonline</tt> flag is set whenever the corresponding +CPU comes online, which means that the debugfs tracing need not dump +out any <tt>rcu_data</tt> structure for which this flag is not set. + +<h5>Quiescent-State and Grace-Period Tracking</h5> + +<p>This portion of the <tt>rcu_data</tt> structure is declared +as follows: + +<pre> + 1 unsigned long completed; + 2 unsigned long gpnum; + 3 bool cpu_no_qs; + 4 bool core_needs_qs; + 5 bool gpwrap; + 6 unsigned long rcu_qs_ctr_snap; +</pre> + +<p>The <tt>completed</tt> and <tt>gpnum</tt> +fields are the counterparts of the fields of the same name +in the <tt>rcu_state</tt> and <tt>rcu_node</tt> structures. +They may each lag up to one behind their <tt>rcu_node</tt> +counterparts, but in <tt>CONFIG_NO_HZ_IDLE</tt> and +<tt>CONFIG_NO_HZ_FULL</tt> kernels can lag +arbitrarily far behind for CPUs in dyntick-idle mode (but these counters +will catch up upon exit from dyntick-idle mode). +If a given <tt>rcu_data</tt> structure's <tt>->gpnum</tt> and +<tt>->complete</tt> fields are equal, then this <tt>rcu_data</tt> +structure believes that RCU is idle. +Otherwise, as with the <tt>rcu_state</tt> and <tt>rcu_node</tt> +structure, +the <tt>->gpnum</tt> field will be one greater than the +<tt>->complete</tt> fields, with <tt>->gpnum</tt> +indicating which grace period this <tt>rcu_data</tt> believes +is still being waited for. + +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + All this replication of the grace period numbers can only cause + massive confusion. + Why not just keep a global pair of counters and be done with it??? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + Because if there was only a single global pair of grace-period + numbers, there would need to be a single global lock to allow + safely accessing and updating them. + And if we are not going to have a single global lock, we need + to carefully manage the numbers on a per-node basis. + Recall from the answer to a previous Quick Quiz that the consequences + of applying a previously sampled quiescent state to the wrong + grace period are quite severe. +</font></td></tr> +<tr><td> </td></tr> +</table> + +<p>The <tt>->cpu_no_qs</tt> flag indicates that the +CPU has not yet passed through a quiescent state, +while the <tt>->core_needs_qs</tt> flag indicates that the +RCU core needs a quiescent state from the corresponding CPU. +The <tt>->gpwrap</tt> field indicates that the corresponding +CPU has remained idle for so long that the <tt>completed</tt> +and <tt>gpnum</tt> counters are in danger of overflow, which +will cause the CPU to disregard the values of its counters on +its next exit from idle. +Finally, the <tt>rcu_qs_ctr_snap</tt> field is used to detect +cases where a given operation has resulted in a quiescent state +for all flavors of RCU, for example, <tt>cond_resched_rcu_qs()</tt>. + +<h5>RCU Callback Handling</h5> + +<p>In the absence of CPU-hotplug events, RCU callbacks are invoked by +the same CPU that registered them. +This is strictly a cache-locality optimization: callbacks can and +do get invoked on CPUs other than the one that registered them. +After all, if the CPU that registered a given callback has gone +offline before the callback can be invoked, there really is no other +choice. + +</p><p>This portion of the <tt>rcu_data</tt> structure is declared +as follows: + +<pre> + 1 struct rcu_head *nxtlist; + 2 struct rcu_head **nxttail[RCU_NEXT_SIZE]; + 3 unsigned long nxtcompleted[RCU_NEXT_SIZE]; + 4 long qlen_lazy; + 5 long qlen; + 6 long qlen_last_fqs_check; + 7 unsigned long n_force_qs_snap; + 8 unsigned long n_cbs_invoked; + 9 unsigned long n_cbs_orphaned; +10 unsigned long n_cbs_adopted; +11 long blimit; +</pre> + +<p>The <tt>->nxtlist</tt> pointer and the +<tt>->nxttail[]</tt> array form a four-segment list with +older callbacks near the head and newer ones near the tail. +Each segment contains callbacks with the corresponding relationship +to the current grace period. +The pointer out of the end of each of the four segments is referenced +by the element of the <tt>->nxttail[]</tt> array indexed by +<tt>RCU_DONE_TAIL</tt> (for callbacks handled by a prior grace period), +<tt>RCU_WAIT_TAIL</tt> (for callbacks waiting on the current grace period), +<tt>RCU_NEXT_READY_TAIL</tt> (for callbacks that will wait on the next +grace period), and +<tt>RCU_NEXT_TAIL</tt> (for callbacks that are not yet associated +with a specific grace period) +respectively, as shown in the following figure. + +</p><p><img src="nxtlist.svg" alt="nxtlist.svg" width="40%"> + +</p><p>In this figure, the <tt>->nxtlist</tt> pointer references the +first +RCU callback in the list. +The <tt>->nxttail[RCU_DONE_TAIL]</tt> array element references +the <tt>->nxtlist</tt> pointer itself, indicating that none +of the callbacks is ready to invoke. +The <tt>->nxttail[RCU_WAIT_TAIL]</tt> array element references callback +CB 2's <tt>->next</tt> pointer, which indicates that +CB 1 and CB 2 are both waiting on the current grace period. +The <tt>->nxttail[RCU_NEXT_READY_TAIL]</tt> array element +references the same RCU callback that <tt>->nxttail[RCU_WAIT_TAIL]</tt> +does, which indicates that there are no callbacks waiting on the next +RCU grace period. +The <tt>->nxttail[RCU_NEXT_TAIL]</tt> array element references +CB 4's <tt>->next</tt> pointer, indicating that all the +remaining RCU callbacks have not yet been assigned to an RCU grace +period. +Note that the <tt>->nxttail[RCU_NEXT_TAIL]</tt> array element +always references the last RCU callback's <tt>->next</tt> pointer +unless the callback list is empty, in which case it references +the <tt>->nxtlist</tt> pointer. + +</p><p>CPUs advance their callbacks from the +<tt>RCU_NEXT_TAIL</tt> to the <tt>RCU_NEXT_READY_TAIL</tt> to the +<tt>RCU_WAIT_TAIL</tt> to the <tt>RCU_DONE_TAIL</tt> list segments +as grace periods advance. +The CPU advances the callbacks in its <tt>rcu_data</tt> structure +whenever it notices that another RCU grace period has completed. +The CPU detects the completion of an RCU grace period by noticing +that the value of its <tt>rcu_data</tt> structure's +<tt>->completed</tt> field differs from that of its leaf +<tt>rcu_node</tt> structure. +Recall that each <tt>rcu_node</tt> structure's +<tt>->completed</tt> field is updated at the end of each +grace period. + +</p><p>The <tt>->nxtcompleted[]</tt> array records grace-period +numbers corresponding to the list segments. +This allows CPUs that go idle for extended periods to determine +which of their callbacks are ready to be invoked after reawakening. + +</p><p>The <tt>->qlen</tt> counter contains the number of +callbacks in <tt>->nxtlist</tt>, and the +<tt>->qlen_lazy</tt> contains the number of those callbacks that +are known to only free memory, and whose invocation can therefore +be safely deferred. +The <tt>->qlen_last_fqs_check</tt> and +<tt>->n_force_qs_snap</tt> coordinate the forcing of quiescent +states from <tt>call_rcu()</tt> and friends when callback +lists grow excessively long. + +</p><p>The <tt>->n_cbs_invoked</tt>, +<tt>->n_cbs_orphaned</tt>, and <tt>->n_cbs_adopted</tt> +fields count the number of callbacks invoked, +sent to other CPUs when this CPU goes offline, +and received from other CPUs when those other CPUs go offline. +Finally, the <tt>->blimit</tt> counter is the maximum number of +RCU callbacks that may be invoked at a given time. + +<h5>Dyntick-Idle Handling</h5> + +<p>This portion of the <tt>rcu_data</tt> structure is declared +as follows: + +<pre> + 1 int dynticks_snap; + 2 unsigned long dynticks_fqs; +</pre> + +The <tt>->dynticks_snap</tt> field is used to take a snapshot +of the corresponding CPU's dyntick-idle state when forcing +quiescent states, and is therefore accessed from other CPUs. +Finally, the <tt>->dynticks_fqs</tt> field is used to +count the number of times this CPU is determined to be in +dyntick-idle state, and is used for tracing and debugging purposes. + +<h3><a name="The rcu_dynticks Structure"> +The <tt>rcu_dynticks</tt> Structure</a></h3> + +<p>The <tt>rcu_dynticks</tt> maintains the per-CPU dyntick-idle state +for the corresponding CPU. +Unlike the other structures, <tt>rcu_dynticks</tt> is not +replicated over the different flavors of RCU. +The fields in this structure may be accessed only from the corresponding +CPU (and from tracing) unless otherwise stated. +Its fields are as follows: + +<pre> + 1 int dynticks_nesting; + 2 int dynticks_nmi_nesting; + 3 atomic_t dynticks; +</pre> + +<p>The <tt>->dynticks_nesting</tt> field counts the +nesting depth of normal interrupts. +In addition, this counter is incremented when exiting dyntick-idle +mode and decremented when entering it. +This counter can therefore be thought of as counting the number +of reasons why this CPU cannot be permitted to enter dyntick-idle +mode, aside from non-maskable interrupts (NMIs). +NMIs are counted by the <tt>->dynticks_nmi_nesting</tt> +field, except that NMIs that interrupt non-dyntick-idle execution +are not counted. + +</p><p>Finally, the <tt>->dynticks</tt> field counts the corresponding +CPU's transitions to and from dyntick-idle mode, so that this counter +has an even value when the CPU is in dyntick-idle mode and an odd +value otherwise. + +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + Why not just count all NMIs? + Wouldn't that be simpler and less error prone? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + It seems simpler only until you think hard about how to go about + updating the <tt>rcu_dynticks</tt> structure's + <tt>->dynticks</tt> field. +</font></td></tr> +<tr><td> </td></tr> +</table> + +<p>Additional fields are present for some special-purpose +builds, and are discussed separately. + +<h3><a name="The rcu_head Structure"> +The <tt>rcu_head</tt> Structure</a></h3> + +<p>Each <tt>rcu_head</tt> structure represents an RCU callback. +These structures are normally embedded within RCU-protected data +structures whose algorithms use asynchronous grace periods. +In contrast, when using algorithms that block waiting for RCU grace periods, +RCU users need not provide <tt>rcu_head</tt> structures. + +</p><p>The <tt>rcu_head</tt> structure has fields as follows: + +<pre> + 1 struct rcu_head *next; + 2 void (*func)(struct rcu_head *head); +</pre> + +<p>The <tt>->next</tt> field is used +to link the <tt>rcu_head</tt> structures together in the +lists within the <tt>rcu_data</tt> structures. +The <tt>->func</tt> field is a pointer to the function +to be called when the callback is ready to be invoked, and +this function is passed a pointer to the <tt>rcu_head</tt> +structure. +However, <tt>kfree_rcu()</tt> uses the <tt>->func</tt> +field to record the offset of the <tt>rcu_head</tt> +structure within the enclosing RCU-protected data structure. + +</p><p>Both of these fields are used internally by RCU. +From the viewpoint of RCU users, this structure is an +opaque “cookie”. + +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + Given that the callback function <tt>->func</tt> + is passed a pointer to the <tt>rcu_head</tt> structure, + how is that function supposed to find the beginning of the + enclosing RCU-protected data structure? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + In actual practice, there is a separate callback function per + type of RCU-protected data structure. + The callback function can therefore use the <tt>container_of()</tt> + macro in the Linux kernel (or other pointer-manipulation facilities + in other software environments) to find the beginning of the + enclosing structure. +</font></td></tr> +<tr><td> </td></tr> +</table> + +<h3><a name="RCU-Specific Fields in the task_struct Structure"> +RCU-Specific Fields in the <tt>task_struct</tt> Structure</a></h3> + +<p>The <tt>CONFIG_PREEMPT_RCU</tt> implementation uses some +additional fields in the <tt>task_struct</tt> structure: + +<pre> + 1 #ifdef CONFIG_PREEMPT_RCU + 2 int rcu_read_lock_nesting; + 3 union rcu_special rcu_read_unlock_special; + 4 struct list_head rcu_node_entry; + 5 struct rcu_node *rcu_blocked_node; + 6 #endif /* #ifdef CONFIG_PREEMPT_RCU */ + 7 #ifdef CONFIG_TASKS_RCU + 8 unsigned long rcu_tasks_nvcsw; + 9 bool rcu_tasks_holdout; +10 struct list_head rcu_tasks_holdout_list; +11 int rcu_tasks_idle_cpu; +12 #endif /* #ifdef CONFIG_TASKS_RCU */ +</pre> + +<p>The <tt>->rcu_read_lock_nesting</tt> field records the +nesting level for RCU read-side critical sections, and +the <tt>->rcu_read_unlock_special</tt> field is a bitmask +that records special conditions that require <tt>rcu_read_unlock()</tt> +to do additional work. +The <tt>->rcu_node_entry</tt> field is used to form lists of +tasks that have blocked within preemptible-RCU read-side critical +sections and the <tt>->rcu_blocked_node</tt> field references +the <tt>rcu_node</tt> structure whose list this task is a member of, +or <tt>NULL</tt> if it is not blocked within a preemptible-RCU +read-side critical section. + +<p>The <tt>->rcu_tasks_nvcsw</tt> field tracks the number of +voluntary context switches that this task had undergone at the +beginning of the current tasks-RCU grace period, +<tt>->rcu_tasks_holdout</tt> is set if the current tasks-RCU +grace period is waiting on this task, <tt>->rcu_tasks_holdout_list</tt> +is a list element enqueuing this task on the holdout list, +and <tt>->rcu_tasks_idle_cpu</tt> tracks which CPU this +idle task is running, but only if the task is currently running, +that is, if the CPU is currently idle. + +<h3><a name="Accessor Functions"> +Accessor Functions</a></h3> + +<p>The following listing shows the +<tt>rcu_get_root()</tt>, <tt>rcu_for_each_node_breadth_first</tt>, +<tt>rcu_for_each_nonleaf_node_breadth_first()</tt>, and +<tt>rcu_for_each_leaf_node()</tt> function and macros: + +<pre> + 1 static struct rcu_node *rcu_get_root(struct rcu_state *rsp) + 2 { + 3 return &rsp->node[0]; + 4 } + 5 + 6 #define rcu_for_each_node_breadth_first(rsp, rnp) \ + 7 for ((rnp) = &(rsp)->node[0]; \ + 8 (rnp) < &(rsp)->node[NUM_RCU_NODES]; (rnp)++) + 9 + 10 #define rcu_for_each_nonleaf_node_breadth_first(rsp, rnp) \ + 11 for ((rnp) = &(rsp)->node[0]; \ + 12 (rnp) < (rsp)->level[NUM_RCU_LVLS - 1]; (rnp)++) + 13 + 14 #define rcu_for_each_leaf_node(rsp, rnp) \ + 15 for ((rnp) = (rsp)->level[NUM_RCU_LVLS - 1]; \ + 16 (rnp) < &(rsp)->node[NUM_RCU_NODES]; (rnp)++) +</pre> + +<p>The <tt>rcu_get_root()</tt> simply returns a pointer to the +first element of the specified <tt>rcu_state</tt> structure's +<tt>->node[]</tt> array, which is the root <tt>rcu_node</tt> +structure. + +</p><p>As noted earlier, the <tt>rcu_for_each_node_breadth_first()</tt> +macro takes advantage of the layout of the <tt>rcu_node</tt> +structures in the <tt>rcu_state</tt> structure's +<tt>->node[]</tt> array, performing a breadth-first traversal by +simply traversing the array in order. +The <tt>rcu_for_each_nonleaf_node_breadth_first()</tt> macro operates +similarly, but traverses only the first part of the array, thus excluding +the leaf <tt>rcu_node</tt> structures. +Finally, the <tt>rcu_for_each_leaf_node()</tt> macro traverses only +the last part of the array, thus traversing only the leaf +<tt>rcu_node</tt> structures. + +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + What do <tt>rcu_for_each_nonleaf_node_breadth_first()</tt> and + <tt>rcu_for_each_leaf_node()</tt> do if the <tt>rcu_node</tt> tree + contains only a single node? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + In the single-node case, + <tt>rcu_for_each_nonleaf_node_breadth_first()</tt> is a no-op + and <tt>rcu_for_each_leaf_node()</tt> traverses the single node. +</font></td></tr> +<tr><td> </td></tr> +</table> + +<h3><a name="Summary"> +Summary</a></h3> + +So each flavor of RCU is represented by an <tt>rcu_state</tt> structure, +which contains a combining tree of <tt>rcu_node</tt> and +<tt>rcu_data</tt> structures. +Finally, in <tt>CONFIG_NO_HZ_IDLE</tt> kernels, each CPU's dyntick-idle +state is tracked by an <tt>rcu_dynticks</tt> structure. + +If you made it this far, you are well prepared to read the code +walkthroughs in the other articles in this series. + +<h3><a name="Acknowledgments"> +Acknowledgments</a></h3> + +I owe thanks to Cyrill Gorcunov, Mathieu Desnoyers, Dhaval Giani, Paul +Turner, Abhishek Srivastava, Matt Kowalczyk, and Serge Hallyn +for helping me get this document into a more human-readable state. + +<h3><a name="Legal Statement"> +Legal Statement</a></h3> + +<p>This work represents the view of the author and does not necessarily +represent the view of IBM. + +</p><p>Linux is a registered trademark of Linus Torvalds. + +</p><p>Other company, product, and service names may be trademarks or +service marks of others. + +</body></html> diff --git a/Documentation/RCU/Design/Data-Structures/HugeTreeClassicRCU.svg b/Documentation/RCU/Design/Data-Structures/HugeTreeClassicRCU.svg new file mode 100644 index 000000000000..2bf12b468206 --- /dev/null +++ b/Documentation/RCU/Design/Data-Structures/HugeTreeClassicRCU.svg @@ -0,0 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font-weight="normal" - font-size="324" - id="text14" - sodipodi:linespacing="125%" - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L" - id="tspan3017">Read-Mostly, Stale &</tspan></text> - <!-- Text --> - <text - xml:space="preserve" - x="7200" - y="1350" - font-style="normal" - font-weight="normal" - font-size="324" - id="text16" - sodipodi:linespacing="125%" - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L" - id="tspan3019">Inconsistent Data OK</tspan></text> - <!-- Text --> - <text - xml:space="preserve" - x="7200" - y="1800" - font-style="normal" - font-weight="normal" - font-size="324" - id="text18" - sodipodi:linespacing="125%" - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L" - id="tspan3021">(RCU Works Great!!!)</tspan></text> - <!-- Text --> - <text - xml:space="preserve" - x="7200" - y="3825" - font-style="normal" - font-weight="normal" - font-size="324" - id="text20" - sodipodi:linespacing="125%" - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L" - id="tspan3023">(RCU Works Well)</tspan></text> - <!-- Text --> - <text - xml:space="preserve" - x="7200" - y="3375" - font-style="normal" - font-weight="normal" - font-size="324" - id="text22" - sodipodi:linespacing="125%" - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L" - id="tspan3025">Read-Mostly, Need Consistent Data</tspan></text> - <!-- Text --> - <text - xml:space="preserve" - x="7200" - y="5175" - font-style="normal" - font-weight="normal" - font-size="324" - id="text24" - sodipodi:linespacing="125%" - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L" - id="tspan3027">Read-Write, Need Consistent Data</tspan></text> - <!-- Text --> - <text - xml:space="preserve" - x="7200" - y="6975" - font-style="normal" - font-weight="normal" - font-size="324" - id="text26" - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L" - sodipodi:linespacing="125%">Update-Mostly, Need Consistent Data</text> - <!-- Text --> - <text - xml:space="preserve" - x="7200" - y="5625" - font-style="normal" - font-weight="normal" - font-size="324" - id="text28" - sodipodi:linespacing="125%" - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L" - id="tspan3029">(RCU Might Be OK...)</tspan></text> - <!-- Text --> - <text - xml:space="preserve" - x="7200" - y="7875" - font-style="normal" - font-weight="normal" - font-size="324" - id="text30" - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L" - sodipodi:linespacing="125%">(1) Provide Existence Guarantees For Update-Friendly Mechanisms</text> - <!-- Text --> - <text - xml:space="preserve" - x="7200" - y="8325" - font-style="normal" - font-weight="normal" - font-size="324" - id="text32" - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L" - sodipodi:linespacing="125%">(2) Provide Wait-Free Read-Side Primitives for Real-Time Use)</text> - <!-- Text --> - <text - xml:space="preserve" - x="7200" - y="7425" - font-style="normal" - font-weight="normal" - font-size="324" - id="text34" - style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L" - sodipodi:linespacing="125%">(RCU is Very Unlikely to be the Right Tool For The Job, But it Can:</text> - </g> -</svg> diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html index a725f9900ec8..e7e24b3e86e2 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.html +++ b/Documentation/RCU/Design/Requirements/Requirements.html @@ -1,5 +1,3 @@ -<!-- DO NOT HAND EDIT. --> -<!-- Instead, edit Documentation/RCU/Design/Requirements/Requirements.htmlx and run 'sh htmlqqz.sh Documentation/RCU/Design/Requirements/Requirements' --> <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> @@ -65,8 +63,8 @@ All that aside, here are the categories of currently known RCU requirements: <p> This is followed by a <a href="#Summary">summary</a>, -which is in turn followed by the inevitable -<a href="#Answers to Quick Quizzes">answers to the quick quizzes</a>. +however, the answers to each quick quiz immediately follows the quiz. +Select the big white space with your mouse to see the answer. <h2><a name="Fundamental Requirements">Fundamental Requirements</a></h2> @@ -153,13 +151,27 @@ Therefore, the outcome: </blockquote> cannot happen. -<p><a name="Quick Quiz 1"><b>Quick Quiz 1</b>:</a> -Wait a minute! -You said that updaters can make useful forward progress concurrently -with readers, but pre-existing readers will block -<tt>synchronize_rcu()</tt>!!! -Just who are you trying to fool??? -<br><a href="#qq1answer">Answer</a> +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + Wait a minute! + You said that updaters can make useful forward progress concurrently + with readers, but pre-existing readers will block + <tt>synchronize_rcu()</tt>!!! + Just who are you trying to fool??? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + First, if updaters do not wish to be blocked by readers, they can use + <tt>call_rcu()</tt> or <tt>kfree_rcu()</tt>, which will + be discussed later. + Second, even when using <tt>synchronize_rcu()</tt>, the other + update-side code does run concurrently with readers, whether + pre-existing or not. +</font></td></tr> +<tr><td> </td></tr> +</table> <p> This scenario resembles one of the first uses of RCU in @@ -210,9 +222,20 @@ to guarantee that <tt>do_something()</tt> never runs concurrently with <tt>recovery()</tt>, but with little or no synchronization overhead in <tt>do_something_dlm()</tt>. -<p><a name="Quick Quiz 2"><b>Quick Quiz 2</b>:</a> -Why is the <tt>synchronize_rcu()</tt> on line 28 needed? -<br><a href="#qq2answer">Answer</a> +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + Why is the <tt>synchronize_rcu()</tt> on line 28 needed? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + Without that extra grace period, memory reordering could result in + <tt>do_something_dlm()</tt> executing <tt>do_something()</tt> + concurrently with the last bits of <tt>recovery()</tt>. +</font></td></tr> +<tr><td> </td></tr> +</table> <p> In order to avoid fatal problems such as deadlocks, @@ -332,12 +355,27 @@ It also prevents any number of “interesting” compiler optimizations, for example, the use of <tt>gp</tt> as a scratch location immediately preceding the assignment. -<p><a name="Quick Quiz 3"><b>Quick Quiz 3</b>:</a> -But <tt>rcu_assign_pointer()</tt> does nothing to prevent the -two assignments to <tt>p->a</tt> and <tt>p->b</tt> -from being reordered. -Can't that also cause problems? -<br><a href="#qq3answer">Answer</a> +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + But <tt>rcu_assign_pointer()</tt> does nothing to prevent the + two assignments to <tt>p->a</tt> and <tt>p->b</tt> + from being reordered. + Can't that also cause problems? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + No, it cannot. + The readers cannot see either of these two fields until + the assignment to <tt>gp</tt>, by which time both fields are + fully initialized. + So reordering the assignments + to <tt>p->a</tt> and <tt>p->b</tt> cannot possibly + cause any problems. +</font></td></tr> +<tr><td> </td></tr> +</table> <p> It is tempting to assume that the reader need not do anything special @@ -494,11 +532,42 @@ The <tt>rcu_access_pointer()</tt> on line 6 is similar to code protected by the corresponding update-side lock. </ol> -<p><a name="Quick Quiz 4"><b>Quick Quiz 4</b>:</a> -Without the <tt>rcu_dereference()</tt> or the -<tt>rcu_access_pointer()</tt>, what destructive optimizations -might the compiler make use of? -<br><a href="#qq4answer">Answer</a> +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + Without the <tt>rcu_dereference()</tt> or the + <tt>rcu_access_pointer()</tt>, what destructive optimizations + might the compiler make use of? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + Let's start with what happens to <tt>do_something_gp()</tt> + if it fails to use <tt>rcu_dereference()</tt>. + It could reuse a value formerly fetched from this same pointer. + It could also fetch the pointer from <tt>gp</tt> in a byte-at-a-time + manner, resulting in <i>load tearing</i>, in turn resulting a bytewise + mash-up of two distince pointer values. + It might even use value-speculation optimizations, where it makes + a wrong guess, but by the time it gets around to checking the + value, an update has changed the pointer to match the wrong guess. + Too bad about any dereferences that returned pre-initialization garbage + in the meantime! + </font> + + <p><font color="ffffff"> + For <tt>remove_gp_synchronous()</tt>, as long as all modifications + to <tt>gp</tt> are carried out while holding <tt>gp_lock</tt>, + the above optimizations are harmless. + However, + with <tt>CONFIG_SPARSE_RCU_POINTER=y</tt>, + <tt>sparse</tt> will complain if you + define <tt>gp</tt> with <tt>__rcu</tt> and then + access it without using + either <tt>rcu_access_pointer()</tt> or <tt>rcu_dereference()</tt>. +</font></td></tr> +<tr><td> </td></tr> +</table> <p> In short, RCU's publish-subscribe guarantee is provided by the combination @@ -571,17 +640,156 @@ systems with more than one CPU: <tt>synchronize_rcu()</tt> migrates in the meantime. </ol> -<p><a name="Quick Quiz 5"><b>Quick Quiz 5</b>:</a> -Given that multiple CPUs can start RCU read-side critical sections -at any time without any ordering whatsoever, how can RCU possibly tell whether -or not a given RCU read-side critical section starts before a -given instance of <tt>synchronize_rcu()</tt>? -<br><a href="#qq5answer">Answer</a> - -<p><a name="Quick Quiz 6"><b>Quick Quiz 6</b>:</a> -The first and second guarantees require unbelievably strict ordering! -Are all these memory barriers <i> really</i> required? -<br><a href="#qq6answer">Answer</a> +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + Given that multiple CPUs can start RCU read-side critical sections + at any time without any ordering whatsoever, how can RCU possibly + tell whether or not a given RCU read-side critical section starts + before a given instance of <tt>synchronize_rcu()</tt>? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + If RCU cannot tell whether or not a given + RCU read-side critical section starts before a + given instance of <tt>synchronize_rcu()</tt>, + then it must assume that the RCU read-side critical section + started first. + In other words, a given instance of <tt>synchronize_rcu()</tt> + can avoid waiting on a given RCU read-side critical section only + if it can prove that <tt>synchronize_rcu()</tt> started first. +</font></td></tr> +<tr><td> </td></tr> +</table> + +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + The first and second guarantees require unbelievably strict ordering! + Are all these memory barriers <i> really</i> required? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + Yes, they really are required. + To see why the first guarantee is required, consider the following + sequence of events: + </font> + + <ol> + <li> <font color="ffffff"> + CPU 1: <tt>rcu_read_lock()</tt> + </font> + <li> <font color="ffffff"> + CPU 1: <tt>q = rcu_dereference(gp); + /* Very likely to return p. */</tt> + </font> + <li> <font color="ffffff"> + CPU 0: <tt>list_del_rcu(p);</tt> + </font> + <li> <font color="ffffff"> + CPU 0: <tt>synchronize_rcu()</tt> starts. + </font> + <li> <font color="ffffff"> + CPU 1: <tt>do_something_with(q->a); + /* No smp_mb(), so might happen after kfree(). */</tt> + </font> + <li> <font color="ffffff"> + CPU 1: <tt>rcu_read_unlock()</tt> + </font> + <li> <font color="ffffff"> + CPU 0: <tt>synchronize_rcu()</tt> returns. + </font> + <li> <font color="ffffff"> + CPU 0: <tt>kfree(p);</tt> + </font> + </ol> + + <p><font color="ffffff"> + Therefore, there absolutely must be a full memory barrier between the + end of the RCU read-side critical section and the end of the + grace period. + </font> + + <p><font color="ffffff"> + The sequence of events demonstrating the necessity of the second rule + is roughly similar: + </font> + + <ol> + <li> <font color="ffffff">CPU 0: <tt>list_del_rcu(p);</tt> + </font> + <li> <font color="ffffff">CPU 0: <tt>synchronize_rcu()</tt> starts. + </font> + <li> <font color="ffffff">CPU 1: <tt>rcu_read_lock()</tt> + </font> + <li> <font color="ffffff">CPU 1: <tt>q = rcu_dereference(gp); + /* Might return p if no memory barrier. */</tt> + </font> + <li> <font color="ffffff">CPU 0: <tt>synchronize_rcu()</tt> returns. + </font> + <li> <font color="ffffff">CPU 0: <tt>kfree(p);</tt> + </font> + <li> <font color="ffffff"> + CPU 1: <tt>do_something_with(q->a); /* Boom!!! */</tt> + </font> + <li> <font color="ffffff">CPU 1: <tt>rcu_read_unlock()</tt> + </font> + </ol> + + <p><font color="ffffff"> + And similarly, without a memory barrier between the beginning of the + grace period and the beginning of the RCU read-side critical section, + CPU 1 might end up accessing the freelist. + </font> + + <p><font color="ffffff"> + The “as if” rule of course applies, so that any + implementation that acts as if the appropriate memory barriers + were in place is a correct implementation. + That said, it is much easier to fool yourself into believing + that you have adhered to the as-if rule than it is to actually + adhere to it! +</font></td></tr> +<tr><td> </td></tr> +</table> + +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + You claim that <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt> + generate absolutely no code in some kernel builds. + This means that the compiler might arbitrarily rearrange consecutive + RCU read-side critical sections. + Given such rearrangement, if a given RCU read-side critical section + is done, how can you be sure that all prior RCU read-side critical + sections are done? + Won't the compiler rearrangements make that impossible to determine? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + In cases where <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt> + generate absolutely no code, RCU infers quiescent states only at + special locations, for example, within the scheduler. + Because calls to <tt>schedule()</tt> had better prevent calling-code + accesses to shared variables from being rearranged across the call to + <tt>schedule()</tt>, if RCU detects the end of a given RCU read-side + critical section, it will necessarily detect the end of all prior + RCU read-side critical sections, no matter how aggressively the + compiler scrambles the code. + </font> + + <p><font color="ffffff"> + Again, this all assumes that the compiler cannot scramble code across + calls to the scheduler, out of interrupt handlers, into the idle loop, + into user-mode code, and so on. + But if your kernel build allows that sort of scrambling, you have broken + far more than just RCU! +</font></td></tr> +<tr><td> </td></tr> +</table> <p> Note that these memory-barrier requirements do not replace the fundamental @@ -626,9 +834,19 @@ inconvenience can be avoided through use of the <tt>call_rcu()</tt> and <tt>kfree_rcu()</tt> API members described later in this document. -<p><a name="Quick Quiz 7"><b>Quick Quiz 7</b>:</a> -But how does the upgrade-to-write operation exclude other readers? -<br><a href="#qq7answer">Answer</a> +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + But how does the upgrade-to-write operation exclude other readers? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + It doesn't, just like normal RCU updates, which also do not exclude + RCU readers. +</font></td></tr> +<tr><td> </td></tr> +</table> <p> This guarantee allows lookup code to be shared between read-side @@ -714,9 +932,20 @@ to do significant reordering. This is by design: Any significant ordering constraints would slow down these fast-path APIs. -<p><a name="Quick Quiz 8"><b>Quick Quiz 8</b>:</a> -Can't the compiler also reorder this code? -<br><a href="#qq8answer">Answer</a> +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + Can't the compiler also reorder this code? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + No, the volatile casts in <tt>READ_ONCE()</tt> and + <tt>WRITE_ONCE()</tt> prevent the compiler from reordering in + this particular case. +</font></td></tr> +<tr><td> </td></tr> +</table> <h3><a name="Readers Do Not Exclude Updaters">Readers Do Not Exclude Updaters</a></h3> @@ -769,10 +998,28 @@ new readers can start immediately after <tt>synchronize_rcu()</tt> starts, and <tt>synchronize_rcu()</tt> is under no obligation to wait for these new readers. -<p><a name="Quick Quiz 9"><b>Quick Quiz 9</b>:</a> -Suppose that synchronize_rcu() did wait until all readers had completed. -Would the updater be able to rely on this? -<br><a href="#qq9answer">Answer</a> +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + Suppose that synchronize_rcu() did wait until <i>all</i> + readers had completed instead of waiting only on + pre-existing readers. + For how long would the updater be able to rely on there + being no readers? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + For no time at all. + Even if <tt>synchronize_rcu()</tt> were to wait until + all readers had completed, a new reader might start immediately after + <tt>synchronize_rcu()</tt> completed. + Therefore, the code following + <tt>synchronize_rcu()</tt> can <i>never</i> rely on there being + no readers. +</font></td></tr> +<tr><td> </td></tr> +</table> <h3><a name="Grace Periods Don't Partition Read-Side Critical Sections"> Grace Periods Don't Partition Read-Side Critical Sections</a></h3> @@ -969,11 +1216,24 @@ grace period. As a result, an RCU read-side critical section cannot partition a pair of RCU grace periods. -<p><a name="Quick Quiz 10"><b>Quick Quiz 10</b>:</a> -How long a sequence of grace periods, each separated by an RCU read-side -critical section, would be required to partition the RCU read-side -critical sections at the beginning and end of the chain? -<br><a href="#qq10answer">Answer</a> +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + How long a sequence of grace periods, each separated by an RCU + read-side critical section, would be required to partition the RCU + read-side critical sections at the beginning and end of the chain? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + In theory, an infinite number. + In practice, an unknown number that is sensitive to both implementation + details and timing considerations. + Therefore, even in practice, RCU users must abide by the + theoretical rather than the practical answer. +</font></td></tr> +<tr><td> </td></tr> +</table> <h3><a name="Disabling Preemption Does Not Block Grace Periods"> Disabling Preemption Does Not Block Grace Periods</a></h3> @@ -1109,12 +1369,27 @@ These classes is covered in the following sections. <h3><a name="Specialization">Specialization</a></h3> <p> -RCU is and always has been intended primarily for read-mostly situations, as -illustrated by the following figure. -This means that RCU's read-side primitives are optimized, often at the +RCU is and always has been intended primarily for read-mostly situations, +which means that RCU's read-side primitives are optimized, often at the expense of its update-side primitives. +Experience thus far is captured by the following list of situations: -<p><img src="RCUApplicability.svg" alt="RCUApplicability.svg" width="70%"></p> +<ol> +<li> Read-mostly data, where stale and inconsistent data is not + a problem: RCU works great! +<li> Read-mostly data, where data must be consistent: + RCU works well. +<li> Read-write data, where data must be consistent: + RCU <i>might</i> work OK. + Or not. +<li> Write-mostly data, where data must be consistent: + RCU is very unlikely to be the right tool for the job, + with the following exceptions, where RCU can provide: + <ol type=a> + <li> Existence guarantees for update-friendly mechanisms. + <li> Wait-free read-side primitives for real-time use. + </ol> +</ol> <p> This focus on read-mostly situations means that RCU must interoperate @@ -1127,9 +1402,43 @@ synchronization primitives be legal within RCU read-side critical sections, including spinlocks, sequence locks, atomic operations, reference counters, and memory barriers. -<p><a name="Quick Quiz 11"><b>Quick Quiz 11</b>:</a> -What about sleeping locks? -<br><a href="#qq11answer">Answer</a> +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + What about sleeping locks? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + These are forbidden within Linux-kernel RCU read-side critical + sections because it is not legal to place a quiescent state + (in this case, voluntary context switch) within an RCU read-side + critical section. + However, sleeping locks may be used within userspace RCU read-side + critical sections, and also within Linux-kernel sleepable RCU + <a href="#Sleepable RCU"><font color="ffffff">(SRCU)</font></a> + read-side critical sections. + In addition, the -rt patchset turns spinlocks into a + sleeping locks so that the corresponding critical sections + can be preempted, which also means that these sleeplockified + spinlocks (but not other sleeping locks!) may be acquire within + -rt-Linux-kernel RCU read-side critical sections. + </font> + + <p><font color="ffffff"> + Note that it <i>is</i> legal for a normal RCU read-side + critical section to conditionally acquire a sleeping locks + (as in <tt>mutex_trylock()</tt>), but only as long as it does + not loop indefinitely attempting to conditionally acquire that + sleeping locks. + The key point is that things like <tt>mutex_trylock()</tt> + either return with the mutex held, or return an error indication if + the mutex was not immediately available. + Either way, <tt>mutex_trylock()</tt> returns immediately without + sleeping. +</font></td></tr> +<tr><td> </td></tr> +</table> <p> It often comes as a surprise that many algorithms do not require a @@ -1160,10 +1469,7 @@ some period of time, so the exact wait period is a judgment call. One of our pair of veternarians might wait 30 seconds before pronouncing the cat dead, while the other might insist on waiting a full minute. The two veternarians would then disagree on the state of the cat during -the final 30 seconds of the minute following the last heartbeat, as -fancifully illustrated below: - -<p><img src="2013-08-is-it-dead.png" alt="2013-08-is-it-dead.png" width="431"></p> +the final 30 seconds of the minute following the last heartbeat. <p> Interestingly enough, this same situation applies to hardware. @@ -1343,7 +1649,8 @@ situations where neither <tt>synchronize_rcu()</tt> nor <tt>synchronize_rcu_expedited()</tt> would be legal, including within preempt-disable code, <tt>local_bh_disable()</tt> code, interrupt-disable code, and interrupt handlers. -However, even <tt>call_rcu()</tt> is illegal within NMI handlers. +However, even <tt>call_rcu()</tt> is illegal within NMI handlers +and from idle and offline CPUs. The callback function (<tt>remove_gp_cb()</tt> in this case) will be executed within softirq (software interrupt) environment within the Linux kernel, @@ -1354,12 +1661,27 @@ write an RCU callback function that takes too long. Long-running operations should be relegated to separate threads or (in the Linux kernel) workqueues. -<p><a name="Quick Quiz 12"><b>Quick Quiz 12</b>:</a> -Why does line 19 use <tt>rcu_access_pointer()</tt>? -After all, <tt>call_rcu()</tt> on line 25 stores into the -structure, which would interact badly with concurrent insertions. -Doesn't this mean that <tt>rcu_dereference()</tt> is required? -<br><a href="#qq12answer">Answer</a> +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + Why does line 19 use <tt>rcu_access_pointer()</tt>? + After all, <tt>call_rcu()</tt> on line 25 stores into the + structure, which would interact badly with concurrent insertions. + Doesn't this mean that <tt>rcu_dereference()</tt> is required? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + Presumably the <tt>->gp_lock</tt> acquired on line 18 excludes + any changes, including any insertions that <tt>rcu_dereference()</tt> + would protect against. + Therefore, any insertions will be delayed until after + <tt>->gp_lock</tt> + is released on line 25, which in turn means that + <tt>rcu_access_pointer()</tt> suffices. +</font></td></tr> +<tr><td> </td></tr> +</table> <p> However, all that <tt>remove_gp_cb()</tt> is doing is @@ -1406,14 +1728,31 @@ This was due to the fact that RCU was not heavily used within DYNIX/ptx, so the very few places that needed something like <tt>synchronize_rcu()</tt> simply open-coded it. -<p><a name="Quick Quiz 13"><b>Quick Quiz 13</b>:</a> -Earlier it was claimed that <tt>call_rcu()</tt> and -<tt>kfree_rcu()</tt> allowed updaters to avoid being blocked -by readers. -But how can that be correct, given that the invocation of the callback -and the freeing of the memory (respectively) must still wait for -a grace period to elapse? -<br><a href="#qq13answer">Answer</a> +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + Earlier it was claimed that <tt>call_rcu()</tt> and + <tt>kfree_rcu()</tt> allowed updaters to avoid being blocked + by readers. + But how can that be correct, given that the invocation of the callback + and the freeing of the memory (respectively) must still wait for + a grace period to elapse? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + We could define things this way, but keep in mind that this sort of + definition would say that updates in garbage-collected languages + cannot complete until the next time the garbage collector runs, + which does not seem at all reasonable. + The key point is that in most cases, an updater using either + <tt>call_rcu()</tt> or <tt>kfree_rcu()</tt> can proceed to the + next update as soon as it has invoked <tt>call_rcu()</tt> or + <tt>kfree_rcu()</tt>, without having to wait for a subsequent + grace period. +</font></td></tr> +<tr><td> </td></tr> +</table> <p> But what if the updater must wait for the completion of code to be @@ -1838,11 +2177,26 @@ kthreads to be spawned. Therefore, invoking <tt>synchronize_rcu()</tt> during scheduler initialization can result in deadlock. -<p><a name="Quick Quiz 14"><b>Quick Quiz 14</b>:</a> -So what happens with <tt>synchronize_rcu()</tt> during -scheduler initialization for <tt>CONFIG_PREEMPT=n</tt> -kernels? -<br><a href="#qq14answer">Answer</a> +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + So what happens with <tt>synchronize_rcu()</tt> during + scheduler initialization for <tt>CONFIG_PREEMPT=n</tt> + kernels? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + In <tt>CONFIG_PREEMPT=n</tt> kernel, <tt>synchronize_rcu()</tt> + maps directly to <tt>synchronize_sched()</tt>. + Therefore, <tt>synchronize_rcu()</tt> works normally throughout + boot in <tt>CONFIG_PREEMPT=n</tt> kernels. + However, your code must also work in <tt>CONFIG_PREEMPT=y</tt> kernels, + so it is still necessary to avoid invoking <tt>synchronize_rcu()</tt> + during scheduler initialization. +</font></td></tr> +<tr><td> </td></tr> +</table> <p> I learned of these boot-time requirements as a result of a series of @@ -2171,6 +2525,14 @@ This real-time requirement motivated the grace-period kthread, which also simplified handling of a number of race conditions. <p> +RCU must avoid degrading real-time response for CPU-bound threads, whether +executing in usermode (which is one use case for +<tt>CONFIG_NO_HZ_FULL=y</tt>) or in the kernel. +That said, CPU-bound loops in the kernel must execute +<tt>cond_resched_rcu_qs()</tt> at least once per few tens of milliseconds +in order to avoid receiving an IPI from RCU. + +<p> Finally, RCU's status as a synchronization primitive means that any RCU failure can result in arbitrary memory corruption that can be extremely difficult to debug. @@ -2223,6 +2585,8 @@ described in a separate section. <li> <a href="#Sched Flavor">Sched Flavor</a> <li> <a href="#Sleepable RCU">Sleepable RCU</a> <li> <a href="#Tasks RCU">Tasks RCU</a> +<li> <a href="#Waiting for Multiple Grace Periods"> + Waiting for Multiple Grace Periods</a> </ol> <h3><a name="Bottom-Half Flavor">Bottom-Half Flavor</a></h3> @@ -2472,6 +2836,94 @@ The tasks-RCU API is quite compact, consisting only of <tt>synchronize_rcu_tasks()</tt>, and <tt>rcu_barrier_tasks()</tt>. +<h3><a name="Waiting for Multiple Grace Periods"> +Waiting for Multiple Grace Periods</a></h3> + +<p> +Perhaps you have an RCU protected data structure that is accessed from +RCU read-side critical sections, from softirq handlers, and from +hardware interrupt handlers. +That is three flavors of RCU, the normal flavor, the bottom-half flavor, +and the sched flavor. +How to wait for a compound grace period? + +<p> +The best approach is usually to “just say no!” and +insert <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt> +around each RCU read-side critical section, regardless of what +environment it happens to be in. +But suppose that some of the RCU read-side critical sections are +on extremely hot code paths, and that use of <tt>CONFIG_PREEMPT=n</tt> +is not a viable option, so that <tt>rcu_read_lock()</tt> and +<tt>rcu_read_unlock()</tt> are not free. +What then? + +<p> +You <i>could</i> wait on all three grace periods in succession, as follows: + +<blockquote> +<pre> + 1 synchronize_rcu(); + 2 synchronize_rcu_bh(); + 3 synchronize_sched(); +</pre> +</blockquote> + +<p> +This works, but triples the update-side latency penalty. +In cases where this is not acceptable, <tt>synchronize_rcu_mult()</tt> +may be used to wait on all three flavors of grace period concurrently: + +<blockquote> +<pre> + 1 synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched); +</pre> +</blockquote> + +<p> +But what if it is necessary to also wait on SRCU? +This can be done as follows: + +<blockquote> +<pre> + 1 static void call_my_srcu(struct rcu_head *head, + 2 void (*func)(struct rcu_head *head)) + 3 { + 4 call_srcu(&my_srcu, head, func); + 5 } + 6 + 7 synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched, call_my_srcu); +</pre> +</blockquote> + +<p> +If you needed to wait on multiple different flavors of SRCU +(but why???), you would need to create a wrapper function resembling +<tt>call_my_srcu()</tt> for each SRCU flavor. + +<table> +<tr><th> </th></tr> +<tr><th align="left">Quick Quiz:</th></tr> +<tr><td> + But what if I need to wait for multiple RCU flavors, but I also need + the grace periods to be expedited? +</td></tr> +<tr><th align="left">Answer:</th></tr> +<tr><td bgcolor="#ffffff"><font color="ffffff"> + If you are using expedited grace periods, there should be less penalty + for waiting on them in succession. + But if that is nevertheless a problem, you can use workqueues + or multiple kthreads to wait on the various expedited grace + periods concurrently. +</font></td></tr> +<tr><td> </td></tr> +</table> + +<p> +Again, it is usually better to adjust the RCU read-side critical sections +to use a single flavor of RCU, but when this is not feasible, you can use +<tt>synchronize_rcu_mult()</tt>. + <h2><a name="Possible Future Changes">Possible Future Changes</a></h2> <p> @@ -2569,329 +3021,4 @@ and is provided under the terms of the Creative Commons Attribution-Share Alike 3.0 United States license. -<h3><a name="Answers to Quick Quizzes"> -Answers to Quick Quizzes</a></h3> - -<a name="qq1answer"></a> -<p><b>Quick Quiz 1</b>: -Wait a minute! -You said that updaters can make useful forward progress concurrently -with readers, but pre-existing readers will block -<tt>synchronize_rcu()</tt>!!! -Just who are you trying to fool??? - - -</p><p><b>Answer</b>: -First, if updaters do not wish to be blocked by readers, they can use -<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt>, which will -be discussed later. -Second, even when using <tt>synchronize_rcu()</tt>, the other -update-side code does run concurrently with readers, whether pre-existing -or not. - - -</p><p><a href="#Quick%20Quiz%201"><b>Back to Quick Quiz 1</b>.</a> - -<a name="qq2answer"></a> -<p><b>Quick Quiz 2</b>: -Why is the <tt>synchronize_rcu()</tt> on line 28 needed? - - -</p><p><b>Answer</b>: -Without that extra grace period, memory reordering could result in -<tt>do_something_dlm()</tt> executing <tt>do_something()</tt> -concurrently with the last bits of <tt>recovery()</tt>. - - -</p><p><a href="#Quick%20Quiz%202"><b>Back to Quick Quiz 2</b>.</a> - -<a name="qq3answer"></a> -<p><b>Quick Quiz 3</b>: -But <tt>rcu_assign_pointer()</tt> does nothing to prevent the -two assignments to <tt>p->a</tt> and <tt>p->b</tt> -from being reordered. -Can't that also cause problems? - - -</p><p><b>Answer</b>: -No, it cannot. -The readers cannot see either of these two fields until -the assignment to <tt>gp</tt>, by which time both fields are -fully initialized. -So reordering the assignments -to <tt>p->a</tt> and <tt>p->b</tt> cannot possibly -cause any problems. - - -</p><p><a href="#Quick%20Quiz%203"><b>Back to Quick Quiz 3</b>.</a> - -<a name="qq4answer"></a> -<p><b>Quick Quiz 4</b>: -Without the <tt>rcu_dereference()</tt> or the -<tt>rcu_access_pointer()</tt>, what destructive optimizations -might the compiler make use of? - - -</p><p><b>Answer</b>: -Let's start with what happens to <tt>do_something_gp()</tt> -if it fails to use <tt>rcu_dereference()</tt>. -It could reuse a value formerly fetched from this same pointer. -It could also fetch the pointer from <tt>gp</tt> in a byte-at-a-time -manner, resulting in <i>load tearing</i>, in turn resulting a bytewise -mash-up of two distince pointer values. -It might even use value-speculation optimizations, where it makes a wrong -guess, but by the time it gets around to checking the value, an update -has changed the pointer to match the wrong guess. -Too bad about any dereferences that returned pre-initialization garbage -in the meantime! - -<p> -For <tt>remove_gp_synchronous()</tt>, as long as all modifications -to <tt>gp</tt> are carried out while holding <tt>gp_lock</tt>, -the above optimizations are harmless. -However, -with <tt>CONFIG_SPARSE_RCU_POINTER=y</tt>, -<tt>sparse</tt> will complain if you -define <tt>gp</tt> with <tt>__rcu</tt> and then -access it without using -either <tt>rcu_access_pointer()</tt> or <tt>rcu_dereference()</tt>. - - -</p><p><a href="#Quick%20Quiz%204"><b>Back to Quick Quiz 4</b>.</a> - -<a name="qq5answer"></a> -<p><b>Quick Quiz 5</b>: -Given that multiple CPUs can start RCU read-side critical sections -at any time without any ordering whatsoever, how can RCU possibly tell whether -or not a given RCU read-side critical section starts before a -given instance of <tt>synchronize_rcu()</tt>? - - -</p><p><b>Answer</b>: -If RCU cannot tell whether or not a given -RCU read-side critical section starts before a -given instance of <tt>synchronize_rcu()</tt>, -then it must assume that the RCU read-side critical section -started first. -In other words, a given instance of <tt>synchronize_rcu()</tt> -can avoid waiting on a given RCU read-side critical section only -if it can prove that <tt>synchronize_rcu()</tt> started first. - - -</p><p><a href="#Quick%20Quiz%205"><b>Back to Quick Quiz 5</b>.</a> - -<a name="qq6answer"></a> -<p><b>Quick Quiz 6</b>: -The first and second guarantees require unbelievably strict ordering! -Are all these memory barriers <i> really</i> required? - - -</p><p><b>Answer</b>: -Yes, they really are required. -To see why the first guarantee is required, consider the following -sequence of events: - -<ol> -<li> CPU 1: <tt>rcu_read_lock()</tt> -<li> CPU 1: <tt>q = rcu_dereference(gp); - /* Very likely to return p. */</tt> -<li> CPU 0: <tt>list_del_rcu(p);</tt> -<li> CPU 0: <tt>synchronize_rcu()</tt> starts. -<li> CPU 1: <tt>do_something_with(q->a); - /* No smp_mb(), so might happen after kfree(). */</tt> -<li> CPU 1: <tt>rcu_read_unlock()</tt> -<li> CPU 0: <tt>synchronize_rcu()</tt> returns. -<li> CPU 0: <tt>kfree(p);</tt> -</ol> - -<p> -Therefore, there absolutely must be a full memory barrier between the -end of the RCU read-side critical section and the end of the -grace period. - -<p> -The sequence of events demonstrating the necessity of the second rule -is roughly similar: - -<ol> -<li> CPU 0: <tt>list_del_rcu(p);</tt> -<li> CPU 0: <tt>synchronize_rcu()</tt> starts. -<li> CPU 1: <tt>rcu_read_lock()</tt> -<li> CPU 1: <tt>q = rcu_dereference(gp); - /* Might return p if no memory barrier. */</tt> -<li> CPU 0: <tt>synchronize_rcu()</tt> returns. -<li> CPU 0: <tt>kfree(p);</tt> -<li> CPU 1: <tt>do_something_with(q->a); /* Boom!!! */</tt> -<li> CPU 1: <tt>rcu_read_unlock()</tt> -</ol> - -<p> -And similarly, without a memory barrier between the beginning of the -grace period and the beginning of the RCU read-side critical section, -CPU 1 might end up accessing the freelist. - -<p> -The “as if” rule of course applies, so that any implementation -that acts as if the appropriate memory barriers were in place is a -correct implementation. -That said, it is much easier to fool yourself into believing that you have -adhered to the as-if rule than it is to actually adhere to it! - - -</p><p><a href="#Quick%20Quiz%206"><b>Back to Quick Quiz 6</b>.</a> - -<a name="qq7answer"></a> -<p><b>Quick Quiz 7</b>: -But how does the upgrade-to-write operation exclude other readers? - - -</p><p><b>Answer</b>: -It doesn't, just like normal RCU updates, which also do not exclude -RCU readers. - - -</p><p><a href="#Quick%20Quiz%207"><b>Back to Quick Quiz 7</b>.</a> - -<a name="qq8answer"></a> -<p><b>Quick Quiz 8</b>: -Can't the compiler also reorder this code? - - -</p><p><b>Answer</b>: -No, the volatile casts in <tt>READ_ONCE()</tt> and -<tt>WRITE_ONCE()</tt> prevent the compiler from reordering in -this particular case. - - -</p><p><a href="#Quick%20Quiz%208"><b>Back to Quick Quiz 8</b>.</a> - -<a name="qq9answer"></a> -<p><b>Quick Quiz 9</b>: -Suppose that synchronize_rcu() did wait until all readers had completed. -Would the updater be able to rely on this? - - -</p><p><b>Answer</b>: -No. -Even if <tt>synchronize_rcu()</tt> were to wait until -all readers had completed, a new reader might start immediately after -<tt>synchronize_rcu()</tt> completed. -Therefore, the code following -<tt>synchronize_rcu()</tt> cannot rely on there being no readers -in any case. - - -</p><p><a href="#Quick%20Quiz%209"><b>Back to Quick Quiz 9</b>.</a> - -<a name="qq10answer"></a> -<p><b>Quick Quiz 10</b>: -How long a sequence of grace periods, each separated by an RCU read-side -critical section, would be required to partition the RCU read-side -critical sections at the beginning and end of the chain? - - -</p><p><b>Answer</b>: -In theory, an infinite number. -In practice, an unknown number that is sensitive to both implementation -details and timing considerations. -Therefore, even in practice, RCU users must abide by the theoretical rather -than the practical answer. - - -</p><p><a href="#Quick%20Quiz%2010"><b>Back to Quick Quiz 10</b>.</a> - -<a name="qq11answer"></a> -<p><b>Quick Quiz 11</b>: -What about sleeping locks? - - -</p><p><b>Answer</b>: -These are forbidden within Linux-kernel RCU read-side critical sections -because it is not legal to place a quiescent state (in this case, -voluntary context switch) within an RCU read-side critical section. -However, sleeping locks may be used within userspace RCU read-side critical -sections, and also within Linux-kernel sleepable RCU -<a href="#Sleepable RCU">(SRCU)</a> -read-side critical sections. -In addition, the -rt patchset turns spinlocks into a sleeping locks so -that the corresponding critical sections can be preempted, which -also means that these sleeplockified spinlocks (but not other sleeping locks!) -may be acquire within -rt-Linux-kernel RCU read-side critical sections. - -<p> -Note that it <i>is</i> legal for a normal RCU read-side critical section -to conditionally acquire a sleeping locks (as in <tt>mutex_trylock()</tt>), -but only as long as it does not loop indefinitely attempting to -conditionally acquire that sleeping locks. -The key point is that things like <tt>mutex_trylock()</tt> -either return with the mutex held, or return an error indication if -the mutex was not immediately available. -Either way, <tt>mutex_trylock()</tt> returns immediately without sleeping. - - -</p><p><a href="#Quick%20Quiz%2011"><b>Back to Quick Quiz 11</b>.</a> - -<a name="qq12answer"></a> -<p><b>Quick Quiz 12</b>: -Why does line 19 use <tt>rcu_access_pointer()</tt>? -After all, <tt>call_rcu()</tt> on line 25 stores into the -structure, which would interact badly with concurrent insertions. -Doesn't this mean that <tt>rcu_dereference()</tt> is required? - - -</p><p><b>Answer</b>: -Presumably the <tt>->gp_lock</tt> acquired on line 18 excludes -any changes, including any insertions that <tt>rcu_dereference()</tt> -would protect against. -Therefore, any insertions will be delayed until after <tt>->gp_lock</tt> -is released on line 25, which in turn means that -<tt>rcu_access_pointer()</tt> suffices. - - -</p><p><a href="#Quick%20Quiz%2012"><b>Back to Quick Quiz 12</b>.</a> - -<a name="qq13answer"></a> -<p><b>Quick Quiz 13</b>: -Earlier it was claimed that <tt>call_rcu()</tt> and -<tt>kfree_rcu()</tt> allowed updaters to avoid being blocked -by readers. -But how can that be correct, given that the invocation of the callback -and the freeing of the memory (respectively) must still wait for -a grace period to elapse? - - -</p><p><b>Answer</b>: -We could define things this way, but keep in mind that this sort of -definition would say that updates in garbage-collected languages -cannot complete until the next time the garbage collector runs, -which does not seem at all reasonable. -The key point is that in most cases, an updater using either -<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt> can proceed to the -next update as soon as it has invoked <tt>call_rcu()</tt> or -<tt>kfree_rcu()</tt>, without having to wait for a subsequent -grace period. - - -</p><p><a href="#Quick%20Quiz%2013"><b>Back to Quick Quiz 13</b>.</a> - -<a name="qq14answer"></a> -<p><b>Quick Quiz 14</b>: -So what happens with <tt>synchronize_rcu()</tt> during -scheduler initialization for <tt>CONFIG_PREEMPT=n</tt> -kernels? - - -</p><p><b>Answer</b>: -In <tt>CONFIG_PREEMPT=n</tt> kernel, <tt>synchronize_rcu()</tt> -maps directly to <tt>synchronize_sched()</tt>. -Therefore, <tt>synchronize_rcu()</tt> works normally throughout -boot in <tt>CONFIG_PREEMPT=n</tt> kernels. -However, your code must also work in <tt>CONFIG_PREEMPT=y</tt> kernels, -so it is still necessary to avoid invoking <tt>synchronize_rcu()</tt> -during scheduler initialization. - - -</p><p><a href="#Quick%20Quiz%2014"><b>Back to Quick Quiz 14</b>.</a> - - </body></html> diff --git a/Documentation/RCU/Design/Requirements/Requirements.htmlx b/Documentation/RCU/Design/Requirements/Requirements.htmlx deleted file mode 100644 index 3a97ba490c42..000000000000 --- a/Documentation/RCU/Design/Requirements/Requirements.htmlx +++ /dev/null @@ -1,2741 +0,0 @@ -<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" - "http://www.w3.org/TR/html4/loose.dtd"> - <html> - <head><title>A Tour Through RCU's Requirements [LWN.net]</title> - <meta HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=utf-8"> - -<h1>A Tour Through RCU's Requirements</h1> - -<p>Copyright IBM Corporation, 2015</p> -<p>Author: Paul E. McKenney</p> -<p><i>The initial version of this document appeared in the -<a href="https://lwn.net/">LWN</a> articles -<a href="https://lwn.net/Articles/652156/">here</a>, -<a href="https://lwn.net/Articles/652677/">here</a>, and -<a href="https://lwn.net/Articles/653326/">here</a>.</i></p> - -<h2>Introduction</h2> - -<p> -Read-copy update (RCU) is a synchronization mechanism that is often -used as a replacement for reader-writer locking. -RCU is unusual in that updaters do not block readers, -which means that RCU's read-side primitives can be exceedingly fast -and scalable. -In addition, updaters can make useful forward progress concurrently -with readers. -However, all this concurrency between RCU readers and updaters does raise -the question of exactly what RCU readers are doing, which in turn -raises the question of exactly what RCU's requirements are. - -<p> -This document therefore summarizes RCU's requirements, and can be thought -of as an informal, high-level specification for RCU. -It is important to understand that RCU's specification is primarily -empirical in nature; -in fact, I learned about many of these requirements the hard way. -This situation might cause some consternation, however, not only -has this learning process been a lot of fun, but it has also been -a great privilege to work with so many people willing to apply -technologies in interesting new ways. - -<p> -All that aside, here are the categories of currently known RCU requirements: -</p> - -<ol> -<li> <a href="#Fundamental Requirements"> - Fundamental Requirements</a> -<li> <a href="#Fundamental Non-Requirements">Fundamental Non-Requirements</a> -<li> <a href="#Parallelism Facts of Life"> - Parallelism Facts of Life</a> -<li> <a href="#Quality-of-Implementation Requirements"> - Quality-of-Implementation Requirements</a> -<li> <a href="#Linux Kernel Complications"> - Linux Kernel Complications</a> -<li> <a href="#Software-Engineering Requirements"> - Software-Engineering Requirements</a> -<li> <a href="#Other RCU Flavors"> - Other RCU Flavors</a> -<li> <a href="#Possible Future Changes"> - Possible Future Changes</a> -</ol> - -<p> -This is followed by a <a href="#Summary">summary</a>, -which is in turn followed by the inevitable -<a href="#Answers to Quick Quizzes">answers to the quick quizzes</a>. - -<h2><a name="Fundamental Requirements">Fundamental Requirements</a></h2> - -<p> -RCU's fundamental requirements are the closest thing RCU has to hard -mathematical requirements. -These are: - -<ol> -<li> <a href="#Grace-Period Guarantee"> - Grace-Period Guarantee</a> -<li> <a href="#Publish-Subscribe Guarantee"> - Publish-Subscribe Guarantee</a> -<li> <a href="#Memory-Barrier Guarantees"> - Memory-Barrier Guarantees</a> -<li> <a href="#RCU Primitives Guaranteed to Execute Unconditionally"> - RCU Primitives Guaranteed to Execute Unconditionally</a> -<li> <a href="#Guaranteed Read-to-Write Upgrade"> - Guaranteed Read-to-Write Upgrade</a> -</ol> - -<h3><a name="Grace-Period Guarantee">Grace-Period Guarantee</a></h3> - -<p> -RCU's grace-period guarantee is unusual in being premeditated: -Jack Slingwine and I had this guarantee firmly in mind when we started -work on RCU (then called “rclock”) in the early 1990s. -That said, the past two decades of experience with RCU have produced -a much more detailed understanding of this guarantee. - -<p> -RCU's grace-period guarantee allows updaters to wait for the completion -of all pre-existing RCU read-side critical sections. -An RCU read-side critical section -begins with the marker <tt>rcu_read_lock()</tt> and ends with -the marker <tt>rcu_read_unlock()</tt>. -These markers may be nested, and RCU treats a nested set as one -big RCU read-side critical section. -Production-quality implementations of <tt>rcu_read_lock()</tt> and -<tt>rcu_read_unlock()</tt> are extremely lightweight, and in -fact have exactly zero overhead in Linux kernels built for production -use with <tt>CONFIG_PREEMPT=n</tt>. - -<p> -This guarantee allows ordering to be enforced with extremely low -overhead to readers, for example: - -<blockquote> -<pre> - 1 int x, y; - 2 - 3 void thread0(void) - 4 { - 5 rcu_read_lock(); - 6 r1 = READ_ONCE(x); - 7 r2 = READ_ONCE(y); - 8 rcu_read_unlock(); - 9 } -10 -11 void thread1(void) -12 { -13 WRITE_ONCE(x, 1); -14 synchronize_rcu(); -15 WRITE_ONCE(y, 1); -16 } -</pre> -</blockquote> - -<p> -Because the <tt>synchronize_rcu()</tt> on line 14 waits for -all pre-existing readers, any instance of <tt>thread0()</tt> that -loads a value of zero from <tt>x</tt> must complete before -<tt>thread1()</tt> stores to <tt>y</tt>, so that instance must -also load a value of zero from <tt>y</tt>. -Similarly, any instance of <tt>thread0()</tt> that loads a value of -one from <tt>y</tt> must have started after the -<tt>synchronize_rcu()</tt> started, and must therefore also load -a value of one from <tt>x</tt>. -Therefore, the outcome: -<blockquote> -<pre> -(r1 == 0 && r2 == 1) -</pre> -</blockquote> -cannot happen. - -<p>@@QQ@@ -Wait a minute! -You said that updaters can make useful forward progress concurrently -with readers, but pre-existing readers will block -<tt>synchronize_rcu()</tt>!!! -Just who are you trying to fool??? -<p>@@QQA@@ -First, if updaters do not wish to be blocked by readers, they can use -<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt>, which will -be discussed later. -Second, even when using <tt>synchronize_rcu()</tt>, the other -update-side code does run concurrently with readers, whether pre-existing -or not. -<p>@@QQE@@ - -<p> -This scenario resembles one of the first uses of RCU in -<a href="https://en.wikipedia.org/wiki/DYNIX">DYNIX/ptx</a>, -which managed a distributed lock manager's transition into -a state suitable for handling recovery from node failure, -more or less as follows: - -<blockquote> -<pre> - 1 #define STATE_NORMAL 0 - 2 #define STATE_WANT_RECOVERY 1 - 3 #define STATE_RECOVERING 2 - 4 #define STATE_WANT_NORMAL 3 - 5 - 6 int state = STATE_NORMAL; - 7 - 8 void do_something_dlm(void) - 9 { -10 int state_snap; -11 -12 rcu_read_lock(); -13 state_snap = READ_ONCE(state); -14 if (state_snap == STATE_NORMAL) -15 do_something(); -16 else -17 do_something_carefully(); -18 rcu_read_unlock(); -19 } -20 -21 void start_recovery(void) -22 { -23 WRITE_ONCE(state, STATE_WANT_RECOVERY); -24 synchronize_rcu(); -25 WRITE_ONCE(state, STATE_RECOVERING); -26 recovery(); -27 WRITE_ONCE(state, STATE_WANT_NORMAL); -28 synchronize_rcu(); -29 WRITE_ONCE(state, STATE_NORMAL); -30 } -</pre> -</blockquote> - -<p> -The RCU read-side critical section in <tt>do_something_dlm()</tt> -works with the <tt>synchronize_rcu()</tt> in <tt>start_recovery()</tt> -to guarantee that <tt>do_something()</tt> never runs concurrently -with <tt>recovery()</tt>, but with little or no synchronization -overhead in <tt>do_something_dlm()</tt>. - -<p>@@QQ@@ -Why is the <tt>synchronize_rcu()</tt> on line 28 needed? -<p>@@QQA@@ -Without that extra grace period, memory reordering could result in -<tt>do_something_dlm()</tt> executing <tt>do_something()</tt> -concurrently with the last bits of <tt>recovery()</tt>. -<p>@@QQE@@ - -<p> -In order to avoid fatal problems such as deadlocks, -an RCU read-side critical section must not contain calls to -<tt>synchronize_rcu()</tt>. -Similarly, an RCU read-side critical section must not -contain anything that waits, directly or indirectly, on completion of -an invocation of <tt>synchronize_rcu()</tt>. - -<p> -Although RCU's grace-period guarantee is useful in and of itself, with -<a href="https://lwn.net/Articles/573497/">quite a few use cases</a>, -it would be good to be able to use RCU to coordinate read-side -access to linked data structures. -For this, the grace-period guarantee is not sufficient, as can -be seen in function <tt>add_gp_buggy()</tt> below. -We will look at the reader's code later, but in the meantime, just think of -the reader as locklessly picking up the <tt>gp</tt> pointer, -and, if the value loaded is non-<tt>NULL</tt>, locklessly accessing the -<tt>->a</tt> and <tt>->b</tt> fields. - -<blockquote> -<pre> - 1 bool add_gp_buggy(int a, int b) - 2 { - 3 p = kmalloc(sizeof(*p), GFP_KERNEL); - 4 if (!p) - 5 return -ENOMEM; - 6 spin_lock(&gp_lock); - 7 if (rcu_access_pointer(gp)) { - 8 spin_unlock(&gp_lock); - 9 return false; -10 } -11 p->a = a; -12 p->b = a; -13 gp = p; /* ORDERING BUG */ -14 spin_unlock(&gp_lock); -15 return true; -16 } -</pre> -</blockquote> - -<p> -The problem is that both the compiler and weakly ordered CPUs are within -their rights to reorder this code as follows: - -<blockquote> -<pre> - 1 bool add_gp_buggy_optimized(int a, int b) - 2 { - 3 p = kmalloc(sizeof(*p), GFP_KERNEL); - 4 if (!p) - 5 return -ENOMEM; - 6 spin_lock(&gp_lock); - 7 if (rcu_access_pointer(gp)) { - 8 spin_unlock(&gp_lock); - 9 return false; -10 } -<b>11 gp = p; /* ORDERING BUG */ -12 p->a = a; -13 p->b = a;</b> -14 spin_unlock(&gp_lock); -15 return true; -16 } -</pre> -</blockquote> - -<p> -If an RCU reader fetches <tt>gp</tt> just after -<tt>add_gp_buggy_optimized</tt> executes line 11, -it will see garbage in the <tt>->a</tt> and <tt>->b</tt> -fields. -And this is but one of many ways in which compiler and hardware optimizations -could cause trouble. -Therefore, we clearly need some way to prevent the compiler and the CPU from -reordering in this manner, which brings us to the publish-subscribe -guarantee discussed in the next section. - -<h3><a name="Publish-Subscribe Guarantee">Publish/Subscribe Guarantee</a></h3> - -<p> -RCU's publish-subscribe guarantee allows data to be inserted -into a linked data structure without disrupting RCU readers. -The updater uses <tt>rcu_assign_pointer()</tt> to insert the -new data, and readers use <tt>rcu_dereference()</tt> to -access data, whether new or old. -The following shows an example of insertion: - -<blockquote> -<pre> - 1 bool add_gp(int a, int b) - 2 { - 3 p = kmalloc(sizeof(*p), GFP_KERNEL); - 4 if (!p) - 5 return -ENOMEM; - 6 spin_lock(&gp_lock); - 7 if (rcu_access_pointer(gp)) { - 8 spin_unlock(&gp_lock); - 9 return false; -10 } -11 p->a = a; -12 p->b = a; -13 rcu_assign_pointer(gp, p); -14 spin_unlock(&gp_lock); -15 return true; -16 } -</pre> -</blockquote> - -<p> -The <tt>rcu_assign_pointer()</tt> on line 13 is conceptually -equivalent to a simple assignment statement, but also guarantees -that its assignment will -happen after the two assignments in lines 11 and 12, -similar to the C11 <tt>memory_order_release</tt> store operation. -It also prevents any number of “interesting” compiler -optimizations, for example, the use of <tt>gp</tt> as a scratch -location immediately preceding the assignment. - -<p>@@QQ@@ -But <tt>rcu_assign_pointer()</tt> does nothing to prevent the -two assignments to <tt>p->a</tt> and <tt>p->b</tt> -from being reordered. -Can't that also cause problems? -<p>@@QQA@@ -No, it cannot. -The readers cannot see either of these two fields until -the assignment to <tt>gp</tt>, by which time both fields are -fully initialized. -So reordering the assignments -to <tt>p->a</tt> and <tt>p->b</tt> cannot possibly -cause any problems. -<p>@@QQE@@ - -<p> -It is tempting to assume that the reader need not do anything special -to control its accesses to the RCU-protected data, -as shown in <tt>do_something_gp_buggy()</tt> below: - -<blockquote> -<pre> - 1 bool do_something_gp_buggy(void) - 2 { - 3 rcu_read_lock(); - 4 p = gp; /* OPTIMIZATIONS GALORE!!! */ - 5 if (p) { - 6 do_something(p->a, p->b); - 7 rcu_read_unlock(); - 8 return true; - 9 } -10 rcu_read_unlock(); -11 return false; -12 } -</pre> -</blockquote> - -<p> -However, this temptation must be resisted because there are a -surprisingly large number of ways that the compiler -(to say nothing of -<a href="https://h71000.www7.hp.com/wizard/wiz_2637.html">DEC Alpha CPUs</a>) -can trip this code up. -For but one example, if the compiler were short of registers, it -might choose to refetch from <tt>gp</tt> rather than keeping -a separate copy in <tt>p</tt> as follows: - -<blockquote> -<pre> - 1 bool do_something_gp_buggy_optimized(void) - 2 { - 3 rcu_read_lock(); - 4 if (gp) { /* OPTIMIZATIONS GALORE!!! */ -<b> 5 do_something(gp->a, gp->b);</b> - 6 rcu_read_unlock(); - 7 return true; - 8 } - 9 rcu_read_unlock(); -10 return false; -11 } -</pre> -</blockquote> - -<p> -If this function ran concurrently with a series of updates that -replaced the current structure with a new one, -the fetches of <tt>gp->a</tt> -and <tt>gp->b</tt> might well come from two different structures, -which could cause serious confusion. -To prevent this (and much else besides), <tt>do_something_gp()</tt> uses -<tt>rcu_dereference()</tt> to fetch from <tt>gp</tt>: - -<blockquote> -<pre> - 1 bool do_something_gp(void) - 2 { - 3 rcu_read_lock(); - 4 p = rcu_dereference(gp); - 5 if (p) { - 6 do_something(p->a, p->b); - 7 rcu_read_unlock(); - 8 return true; - 9 } -10 rcu_read_unlock(); -11 return false; -12 } -</pre> -</blockquote> - -<p> -The <tt>rcu_dereference()</tt> uses volatile casts and (for DEC Alpha) -memory barriers in the Linux kernel. -Should a -<a href="http://www.rdrop.com/users/paulmck/RCU/consume.2015.07.13a.pdf">high-quality implementation of C11 <tt>memory_order_consume</tt> [PDF]</a> -ever appear, then <tt>rcu_dereference()</tt> could be implemented -as a <tt>memory_order_consume</tt> load. -Regardless of the exact implementation, a pointer fetched by -<tt>rcu_dereference()</tt> may not be used outside of the -outermost RCU read-side critical section containing that -<tt>rcu_dereference()</tt>, unless protection of -the corresponding data element has been passed from RCU to some -other synchronization mechanism, most commonly locking or -<a href="https://www.kernel.org/doc/Documentation/RCU/rcuref.txt">reference counting</a>. - -<p> -In short, updaters use <tt>rcu_assign_pointer()</tt> and readers -use <tt>rcu_dereference()</tt>, and these two RCU API elements -work together to ensure that readers have a consistent view of -newly added data elements. - -<p> -Of course, it is also necessary to remove elements from RCU-protected -data structures, for example, using the following process: - -<ol> -<li> Remove the data element from the enclosing structure. -<li> Wait for all pre-existing RCU read-side critical sections - to complete (because only pre-existing readers can possibly have - a reference to the newly removed data element). -<li> At this point, only the updater has a reference to the - newly removed data element, so it can safely reclaim - the data element, for example, by passing it to <tt>kfree()</tt>. -</ol> - -This process is implemented by <tt>remove_gp_synchronous()</tt>: - -<blockquote> -<pre> - 1 bool remove_gp_synchronous(void) - 2 { - 3 struct foo *p; - 4 - 5 spin_lock(&gp_lock); - 6 p = rcu_access_pointer(gp); - 7 if (!p) { - 8 spin_unlock(&gp_lock); - 9 return false; -10 } -11 rcu_assign_pointer(gp, NULL); -12 spin_unlock(&gp_lock); -13 synchronize_rcu(); -14 kfree(p); -15 return true; -16 } -</pre> -</blockquote> - -<p> -This function is straightforward, with line 13 waiting for a grace -period before line 14 frees the old data element. -This waiting ensures that readers will reach line 7 of -<tt>do_something_gp()</tt> before the data element referenced by -<tt>p</tt> is freed. -The <tt>rcu_access_pointer()</tt> on line 6 is similar to -<tt>rcu_dereference()</tt>, except that: - -<ol> -<li> The value returned by <tt>rcu_access_pointer()</tt> - cannot be dereferenced. - If you want to access the value pointed to as well as - the pointer itself, use <tt>rcu_dereference()</tt> - instead of <tt>rcu_access_pointer()</tt>. -<li> The call to <tt>rcu_access_pointer()</tt> need not be - protected. - In contrast, <tt>rcu_dereference()</tt> must either be - within an RCU read-side critical section or in a code - segment where the pointer cannot change, for example, in - code protected by the corresponding update-side lock. -</ol> - -<p>@@QQ@@ -Without the <tt>rcu_dereference()</tt> or the -<tt>rcu_access_pointer()</tt>, what destructive optimizations -might the compiler make use of? -<p>@@QQA@@ -Let's start with what happens to <tt>do_something_gp()</tt> -if it fails to use <tt>rcu_dereference()</tt>. -It could reuse a value formerly fetched from this same pointer. -It could also fetch the pointer from <tt>gp</tt> in a byte-at-a-time -manner, resulting in <i>load tearing</i>, in turn resulting a bytewise -mash-up of two distince pointer values. -It might even use value-speculation optimizations, where it makes a wrong -guess, but by the time it gets around to checking the value, an update -has changed the pointer to match the wrong guess. -Too bad about any dereferences that returned pre-initialization garbage -in the meantime! - -<p> -For <tt>remove_gp_synchronous()</tt>, as long as all modifications -to <tt>gp</tt> are carried out while holding <tt>gp_lock</tt>, -the above optimizations are harmless. -However, -with <tt>CONFIG_SPARSE_RCU_POINTER=y</tt>, -<tt>sparse</tt> will complain if you -define <tt>gp</tt> with <tt>__rcu</tt> and then -access it without using -either <tt>rcu_access_pointer()</tt> or <tt>rcu_dereference()</tt>. -<p>@@QQE@@ - -<p> -In short, RCU's publish-subscribe guarantee is provided by the combination -of <tt>rcu_assign_pointer()</tt> and <tt>rcu_dereference()</tt>. -This guarantee allows data elements to be safely added to RCU-protected -linked data structures without disrupting RCU readers. -This guarantee can be used in combination with the grace-period -guarantee to also allow data elements to be removed from RCU-protected -linked data structures, again without disrupting RCU readers. - -<p> -This guarantee was only partially premeditated. -DYNIX/ptx used an explicit memory barrier for publication, but had nothing -resembling <tt>rcu_dereference()</tt> for subscription, nor did it -have anything resembling the <tt>smp_read_barrier_depends()</tt> -that was later subsumed into <tt>rcu_dereference()</tt>. -The need for these operations made itself known quite suddenly at a -late-1990s meeting with the DEC Alpha architects, back in the days when -DEC was still a free-standing company. -It took the Alpha architects a good hour to convince me that any sort -of barrier would ever be needed, and it then took me a good <i>two</i> hours -to convince them that their documentation did not make this point clear. -More recent work with the C and C++ standards committees have provided -much education on tricks and traps from the compiler. -In short, compilers were much less tricky in the early 1990s, but in -2015, don't even think about omitting <tt>rcu_dereference()</tt>! - -<h3><a name="Memory-Barrier Guarantees">Memory-Barrier Guarantees</a></h3> - -<p> -The previous section's simple linked-data-structure scenario clearly -demonstrates the need for RCU's stringent memory-ordering guarantees on -systems with more than one CPU: - -<ol> -<li> Each CPU that has an RCU read-side critical section that - begins before <tt>synchronize_rcu()</tt> starts is - guaranteed to execute a full memory barrier between the time - that the RCU read-side critical section ends and the time that - <tt>synchronize_rcu()</tt> returns. - Without this guarantee, a pre-existing RCU read-side critical section - might hold a reference to the newly removed <tt>struct foo</tt> - after the <tt>kfree()</tt> on line 14 of - <tt>remove_gp_synchronous()</tt>. -<li> Each CPU that has an RCU read-side critical section that ends - after <tt>synchronize_rcu()</tt> returns is guaranteed - to execute a full memory barrier between the time that - <tt>synchronize_rcu()</tt> begins and the time that the RCU - read-side critical section begins. - Without this guarantee, a later RCU read-side critical section - running after the <tt>kfree()</tt> on line 14 of - <tt>remove_gp_synchronous()</tt> might - later run <tt>do_something_gp()</tt> and find the - newly deleted <tt>struct foo</tt>. -<li> If the task invoking <tt>synchronize_rcu()</tt> remains - on a given CPU, then that CPU is guaranteed to execute a full - memory barrier sometime during the execution of - <tt>synchronize_rcu()</tt>. - This guarantee ensures that the <tt>kfree()</tt> on - line 14 of <tt>remove_gp_synchronous()</tt> really does - execute after the removal on line 11. -<li> If the task invoking <tt>synchronize_rcu()</tt> migrates - among a group of CPUs during that invocation, then each of the - CPUs in that group is guaranteed to execute a full memory barrier - sometime during the execution of <tt>synchronize_rcu()</tt>. - This guarantee also ensures that the <tt>kfree()</tt> on - line 14 of <tt>remove_gp_synchronous()</tt> really does - execute after the removal on - line 11, but also in the case where the thread executing the - <tt>synchronize_rcu()</tt> migrates in the meantime. -</ol> - -<p>@@QQ@@ -Given that multiple CPUs can start RCU read-side critical sections -at any time without any ordering whatsoever, how can RCU possibly tell whether -or not a given RCU read-side critical section starts before a -given instance of <tt>synchronize_rcu()</tt>? -<p>@@QQA@@ -If RCU cannot tell whether or not a given -RCU read-side critical section starts before a -given instance of <tt>synchronize_rcu()</tt>, -then it must assume that the RCU read-side critical section -started first. -In other words, a given instance of <tt>synchronize_rcu()</tt> -can avoid waiting on a given RCU read-side critical section only -if it can prove that <tt>synchronize_rcu()</tt> started first. -<p>@@QQE@@ - -<p>@@QQ@@ -The first and second guarantees require unbelievably strict ordering! -Are all these memory barriers <i> really</i> required? -<p>@@QQA@@ -Yes, they really are required. -To see why the first guarantee is required, consider the following -sequence of events: - -<ol> -<li> CPU 1: <tt>rcu_read_lock()</tt> -<li> CPU 1: <tt>q = rcu_dereference(gp); - /* Very likely to return p. */</tt> -<li> CPU 0: <tt>list_del_rcu(p);</tt> -<li> CPU 0: <tt>synchronize_rcu()</tt> starts. -<li> CPU 1: <tt>do_something_with(q->a); - /* No smp_mb(), so might happen after kfree(). */</tt> -<li> CPU 1: <tt>rcu_read_unlock()</tt> -<li> CPU 0: <tt>synchronize_rcu()</tt> returns. -<li> CPU 0: <tt>kfree(p);</tt> -</ol> - -<p> -Therefore, there absolutely must be a full memory barrier between the -end of the RCU read-side critical section and the end of the -grace period. - -<p> -The sequence of events demonstrating the necessity of the second rule -is roughly similar: - -<ol> -<li> CPU 0: <tt>list_del_rcu(p);</tt> -<li> CPU 0: <tt>synchronize_rcu()</tt> starts. -<li> CPU 1: <tt>rcu_read_lock()</tt> -<li> CPU 1: <tt>q = rcu_dereference(gp); - /* Might return p if no memory barrier. */</tt> -<li> CPU 0: <tt>synchronize_rcu()</tt> returns. -<li> CPU 0: <tt>kfree(p);</tt> -<li> CPU 1: <tt>do_something_with(q->a); /* Boom!!! */</tt> -<li> CPU 1: <tt>rcu_read_unlock()</tt> -</ol> - -<p> -And similarly, without a memory barrier between the beginning of the -grace period and the beginning of the RCU read-side critical section, -CPU 1 might end up accessing the freelist. - -<p> -The “as if” rule of course applies, so that any implementation -that acts as if the appropriate memory barriers were in place is a -correct implementation. -That said, it is much easier to fool yourself into believing that you have -adhered to the as-if rule than it is to actually adhere to it! -<p>@@QQE@@ - -<p> -Note that these memory-barrier requirements do not replace the fundamental -RCU requirement that a grace period wait for all pre-existing readers. -On the contrary, the memory barriers called out in this section must operate in -such a way as to <i>enforce</i> this fundamental requirement. -Of course, different implementations enforce this requirement in different -ways, but enforce it they must. - -<h3><a name="RCU Primitives Guaranteed to Execute Unconditionally">RCU Primitives Guaranteed to Execute Unconditionally</a></h3> - -<p> -The common-case RCU primitives are unconditional. -They are invoked, they do their job, and they return, with no possibility -of error, and no need to retry. -This is a key RCU design philosophy. - -<p> -However, this philosophy is pragmatic rather than pigheaded. -If someone comes up with a good justification for a particular conditional -RCU primitive, it might well be implemented and added. -After all, this guarantee was reverse-engineered, not premeditated. -The unconditional nature of the RCU primitives was initially an -accident of implementation, and later experience with synchronization -primitives with conditional primitives caused me to elevate this -accident to a guarantee. -Therefore, the justification for adding a conditional primitive to -RCU would need to be based on detailed and compelling use cases. - -<h3><a name="Guaranteed Read-to-Write Upgrade">Guaranteed Read-to-Write Upgrade</a></h3> - -<p> -As far as RCU is concerned, it is always possible to carry out an -update within an RCU read-side critical section. -For example, that RCU read-side critical section might search for -a given data element, and then might acquire the update-side -spinlock in order to update that element, all while remaining -in that RCU read-side critical section. -Of course, it is necessary to exit the RCU read-side critical section -before invoking <tt>synchronize_rcu()</tt>, however, this -inconvenience can be avoided through use of the -<tt>call_rcu()</tt> and <tt>kfree_rcu()</tt> API members -described later in this document. - -<p>@@QQ@@ -But how does the upgrade-to-write operation exclude other readers? -<p>@@QQA@@ -It doesn't, just like normal RCU updates, which also do not exclude -RCU readers. -<p>@@QQE@@ - -<p> -This guarantee allows lookup code to be shared between read-side -and update-side code, and was premeditated, appearing in the earliest -DYNIX/ptx RCU documentation. - -<h2><a name="Fundamental Non-Requirements">Fundamental Non-Requirements</a></h2> - -<p> -RCU provides extremely lightweight readers, and its read-side guarantees, -though quite useful, are correspondingly lightweight. -It is therefore all too easy to assume that RCU is guaranteeing more -than it really is. -Of course, the list of things that RCU does not guarantee is infinitely -long, however, the following sections list a few non-guarantees that -have caused confusion. -Except where otherwise noted, these non-guarantees were premeditated. - -<ol> -<li> <a href="#Readers Impose Minimal Ordering"> - Readers Impose Minimal Ordering</a> -<li> <a href="#Readers Do Not Exclude Updaters"> - Readers Do Not Exclude Updaters</a> -<li> <a href="#Updaters Only Wait For Old Readers"> - Updaters Only Wait For Old Readers</a> -<li> <a href="#Grace Periods Don't Partition Read-Side Critical Sections"> - Grace Periods Don't Partition Read-Side Critical Sections</a> -<li> <a href="#Read-Side Critical Sections Don't Partition Grace Periods"> - Read-Side Critical Sections Don't Partition Grace Periods</a> -<li> <a href="#Disabling Preemption Does Not Block Grace Periods"> - Disabling Preemption Does Not Block Grace Periods</a> -</ol> - -<h3><a name="Readers Impose Minimal Ordering">Readers Impose Minimal Ordering</a></h3> - -<p> -Reader-side markers such as <tt>rcu_read_lock()</tt> and -<tt>rcu_read_unlock()</tt> provide absolutely no ordering guarantees -except through their interaction with the grace-period APIs such as -<tt>synchronize_rcu()</tt>. -To see this, consider the following pair of threads: - -<blockquote> -<pre> - 1 void thread0(void) - 2 { - 3 rcu_read_lock(); - 4 WRITE_ONCE(x, 1); - 5 rcu_read_unlock(); - 6 rcu_read_lock(); - 7 WRITE_ONCE(y, 1); - 8 rcu_read_unlock(); - 9 } -10 -11 void thread1(void) -12 { -13 rcu_read_lock(); -14 r1 = READ_ONCE(y); -15 rcu_read_unlock(); -16 rcu_read_lock(); -17 r2 = READ_ONCE(x); -18 rcu_read_unlock(); -19 } -</pre> -</blockquote> - -<p> -After <tt>thread0()</tt> and <tt>thread1()</tt> execute -concurrently, it is quite possible to have - -<blockquote> -<pre> -(r1 == 1 && r2 == 0) -</pre> -</blockquote> - -(that is, <tt>y</tt> appears to have been assigned before <tt>x</tt>), -which would not be possible if <tt>rcu_read_lock()</tt> and -<tt>rcu_read_unlock()</tt> had much in the way of ordering -properties. -But they do not, so the CPU is within its rights -to do significant reordering. -This is by design: Any significant ordering constraints would slow down -these fast-path APIs. - -<p>@@QQ@@ -Can't the compiler also reorder this code? -<p>@@QQA@@ -No, the volatile casts in <tt>READ_ONCE()</tt> and -<tt>WRITE_ONCE()</tt> prevent the compiler from reordering in -this particular case. -<p>@@QQE@@ - -<h3><a name="Readers Do Not Exclude Updaters">Readers Do Not Exclude Updaters</a></h3> - -<p> -Neither <tt>rcu_read_lock()</tt> nor <tt>rcu_read_unlock()</tt> -exclude updates. -All they do is to prevent grace periods from ending. -The following example illustrates this: - -<blockquote> -<pre> - 1 void thread0(void) - 2 { - 3 rcu_read_lock(); - 4 r1 = READ_ONCE(y); - 5 if (r1) { - 6 do_something_with_nonzero_x(); - 7 r2 = READ_ONCE(x); - 8 WARN_ON(!r2); /* BUG!!! */ - 9 } -10 rcu_read_unlock(); -11 } -12 -13 void thread1(void) -14 { -15 spin_lock(&my_lock); -16 WRITE_ONCE(x, 1); -17 WRITE_ONCE(y, 1); -18 spin_unlock(&my_lock); -19 } -</pre> -</blockquote> - -<p> -If the <tt>thread0()</tt> function's <tt>rcu_read_lock()</tt> -excluded the <tt>thread1()</tt> function's update, -the <tt>WARN_ON()</tt> could never fire. -But the fact is that <tt>rcu_read_lock()</tt> does not exclude -much of anything aside from subsequent grace periods, of which -<tt>thread1()</tt> has none, so the -<tt>WARN_ON()</tt> can and does fire. - -<h3><a name="Updaters Only Wait For Old Readers">Updaters Only Wait For Old Readers</a></h3> - -<p> -It might be tempting to assume that after <tt>synchronize_rcu()</tt> -completes, there are no readers executing. -This temptation must be avoided because -new readers can start immediately after <tt>synchronize_rcu()</tt> -starts, and <tt>synchronize_rcu()</tt> is under no -obligation to wait for these new readers. - -<p>@@QQ@@ -Suppose that synchronize_rcu() did wait until all readers had completed. -Would the updater be able to rely on this? -<p>@@QQA@@ -No. -Even if <tt>synchronize_rcu()</tt> were to wait until -all readers had completed, a new reader might start immediately after -<tt>synchronize_rcu()</tt> completed. -Therefore, the code following -<tt>synchronize_rcu()</tt> cannot rely on there being no readers -in any case. -<p>@@QQE@@ - -<h3><a name="Grace Periods Don't Partition Read-Side Critical Sections"> -Grace Periods Don't Partition Read-Side Critical Sections</a></h3> - -<p> -It is tempting to assume that if any part of one RCU read-side critical -section precedes a given grace period, and if any part of another RCU -read-side critical section follows that same grace period, then all of -the first RCU read-side critical section must precede all of the second. -However, this just isn't the case: A single grace period does not -partition the set of RCU read-side critical sections. -An example of this situation can be illustrated as follows, where -<tt>x</tt>, <tt>y</tt>, and <tt>z</tt> are initially all zero: - -<blockquote> -<pre> - 1 void thread0(void) - 2 { - 3 rcu_read_lock(); - 4 WRITE_ONCE(a, 1); - 5 WRITE_ONCE(b, 1); - 6 rcu_read_unlock(); - 7 } - 8 - 9 void thread1(void) -10 { -11 r1 = READ_ONCE(a); -12 synchronize_rcu(); -13 WRITE_ONCE(c, 1); -14 } -15 -16 void thread2(void) -17 { -18 rcu_read_lock(); -19 r2 = READ_ONCE(b); -20 r3 = READ_ONCE(c); -21 rcu_read_unlock(); -22 } -</pre> -</blockquote> - -<p> -It turns out that the outcome: - -<blockquote> -<pre> -(r1 == 1 && r2 == 0 && r3 == 1) -</pre> -</blockquote> - -is entirely possible. -The following figure show how this can happen, with each circled -<tt>QS</tt> indicating the point at which RCU recorded a -<i>quiescent state</i> for each thread, that is, a state in which -RCU knows that the thread cannot be in the midst of an RCU read-side -critical section that started before the current grace period: - -<p><img src="GPpartitionReaders1.svg" alt="GPpartitionReaders1.svg" width="60%"></p> - -<p> -If it is necessary to partition RCU read-side critical sections in this -manner, it is necessary to use two grace periods, where the first -grace period is known to end before the second grace period starts: - -<blockquote> -<pre> - 1 void thread0(void) - 2 { - 3 rcu_read_lock(); - 4 WRITE_ONCE(a, 1); - 5 WRITE_ONCE(b, 1); - 6 rcu_read_unlock(); - 7 } - 8 - 9 void thread1(void) -10 { -11 r1 = READ_ONCE(a); -12 synchronize_rcu(); -13 WRITE_ONCE(c, 1); -14 } -15 -16 void thread2(void) -17 { -18 r2 = READ_ONCE(c); -19 synchronize_rcu(); -20 WRITE_ONCE(d, 1); -21 } -22 -23 void thread3(void) -24 { -25 rcu_read_lock(); -26 r3 = READ_ONCE(b); -27 r4 = READ_ONCE(d); -28 rcu_read_unlock(); -29 } -</pre> -</blockquote> - -<p> -Here, if <tt>(r1 == 1)</tt>, then -<tt>thread0()</tt>'s write to <tt>b</tt> must happen -before the end of <tt>thread1()</tt>'s grace period. -If in addition <tt>(r4 == 1)</tt>, then -<tt>thread3()</tt>'s read from <tt>b</tt> must happen -after the beginning of <tt>thread2()</tt>'s grace period. -If it is also the case that <tt>(r2 == 1)</tt>, then the -end of <tt>thread1()</tt>'s grace period must precede the -beginning of <tt>thread2()</tt>'s grace period. -This mean that the two RCU read-side critical sections cannot overlap, -guaranteeing that <tt>(r3 == 1)</tt>. -As a result, the outcome: - -<blockquote> -<pre> -(r1 == 1 && r2 == 1 && r3 == 0 && r4 == 1) -</pre> -</blockquote> - -cannot happen. - -<p> -This non-requirement was also non-premeditated, but became apparent -when studying RCU's interaction with memory ordering. - -<h3><a name="Read-Side Critical Sections Don't Partition Grace Periods"> -Read-Side Critical Sections Don't Partition Grace Periods</a></h3> - -<p> -It is also tempting to assume that if an RCU read-side critical section -happens between a pair of grace periods, then those grace periods cannot -overlap. -However, this temptation leads nowhere good, as can be illustrated by -the following, with all variables initially zero: - -<blockquote> -<pre> - 1 void thread0(void) - 2 { - 3 rcu_read_lock(); - 4 WRITE_ONCE(a, 1); - 5 WRITE_ONCE(b, 1); - 6 rcu_read_unlock(); - 7 } - 8 - 9 void thread1(void) -10 { -11 r1 = READ_ONCE(a); -12 synchronize_rcu(); -13 WRITE_ONCE(c, 1); -14 } -15 -16 void thread2(void) -17 { -18 rcu_read_lock(); -19 WRITE_ONCE(d, 1); -20 r2 = READ_ONCE(c); -21 rcu_read_unlock(); -22 } -23 -24 void thread3(void) -25 { -26 r3 = READ_ONCE(d); -27 synchronize_rcu(); -28 WRITE_ONCE(e, 1); -29 } -30 -31 void thread4(void) -32 { -33 rcu_read_lock(); -34 r4 = READ_ONCE(b); -35 r5 = READ_ONCE(e); -36 rcu_read_unlock(); -37 } -</pre> -</blockquote> - -<p> -In this case, the outcome: - -<blockquote> -<pre> -(r1 == 1 && r2 == 1 && r3 == 1 && r4 == 0 && r5 == 1) -</pre> -</blockquote> - -is entirely possible, as illustrated below: - -<p><img src="ReadersPartitionGP1.svg" alt="ReadersPartitionGP1.svg" width="100%"></p> - -<p> -Again, an RCU read-side critical section can overlap almost all of a -given grace period, just so long as it does not overlap the entire -grace period. -As a result, an RCU read-side critical section cannot partition a pair -of RCU grace periods. - -<p>@@QQ@@ -How long a sequence of grace periods, each separated by an RCU read-side -critical section, would be required to partition the RCU read-side -critical sections at the beginning and end of the chain? -<p>@@QQA@@ -In theory, an infinite number. -In practice, an unknown number that is sensitive to both implementation -details and timing considerations. -Therefore, even in practice, RCU users must abide by the theoretical rather -than the practical answer. -<p>@@QQE@@ - -<h3><a name="Disabling Preemption Does Not Block Grace Periods"> -Disabling Preemption Does Not Block Grace Periods</a></h3> - -<p> -There was a time when disabling preemption on any given CPU would block -subsequent grace periods. -However, this was an accident of implementation and is not a requirement. -And in the current Linux-kernel implementation, disabling preemption -on a given CPU in fact does not block grace periods, as Oleg Nesterov -<a href="https://lkml.kernel.org/g/20150614193825.GA19582@redhat.com">demonstrated</a>. - -<p> -If you need a preempt-disable region to block grace periods, you need to add -<tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>, for example -as follows: - -<blockquote> -<pre> - 1 preempt_disable(); - 2 rcu_read_lock(); - 3 do_something(); - 4 rcu_read_unlock(); - 5 preempt_enable(); - 6 - 7 /* Spinlocks implicitly disable preemption. */ - 8 spin_lock(&mylock); - 9 rcu_read_lock(); -10 do_something(); -11 rcu_read_unlock(); -12 spin_unlock(&mylock); -</pre> -</blockquote> - -<p> -In theory, you could enter the RCU read-side critical section first, -but it is more efficient to keep the entire RCU read-side critical -section contained in the preempt-disable region as shown above. -Of course, RCU read-side critical sections that extend outside of -preempt-disable regions will work correctly, but such critical sections -can be preempted, which forces <tt>rcu_read_unlock()</tt> to do -more work. -And no, this is <i>not</i> an invitation to enclose all of your RCU -read-side critical sections within preempt-disable regions, because -doing so would degrade real-time response. - -<p> -This non-requirement appeared with preemptible RCU. -If you need a grace period that waits on non-preemptible code regions, use -<a href="#Sched Flavor">RCU-sched</a>. - -<h2><a name="Parallelism Facts of Life">Parallelism Facts of Life</a></h2> - -<p> -These parallelism facts of life are by no means specific to RCU, but -the RCU implementation must abide by them. -They therefore bear repeating: - -<ol> -<li> Any CPU or task may be delayed at any time, - and any attempts to avoid these delays by disabling - preemption, interrupts, or whatever are completely futile. - This is most obvious in preemptible user-level - environments and in virtualized environments (where - a given guest OS's VCPUs can be preempted at any time by - the underlying hypervisor), but can also happen in bare-metal - environments due to ECC errors, NMIs, and other hardware - events. - Although a delay of more than about 20 seconds can result - in splats, the RCU implementation is obligated to use - algorithms that can tolerate extremely long delays, but where - “extremely long” is not long enough to allow - wrap-around when incrementing a 64-bit counter. -<li> Both the compiler and the CPU can reorder memory accesses. - Where it matters, RCU must use compiler directives and - memory-barrier instructions to preserve ordering. -<li> Conflicting writes to memory locations in any given cache line - will result in expensive cache misses. - Greater numbers of concurrent writes and more-frequent - concurrent writes will result in more dramatic slowdowns. - RCU is therefore obligated to use algorithms that have - sufficient locality to avoid significant performance and - scalability problems. -<li> As a rough rule of thumb, only one CPU's worth of processing - may be carried out under the protection of any given exclusive - lock. - RCU must therefore use scalable locking designs. -<li> Counters are finite, especially on 32-bit systems. - RCU's use of counters must therefore tolerate counter wrap, - or be designed such that counter wrap would take way more - time than a single system is likely to run. - An uptime of ten years is quite possible, a runtime - of a century much less so. - As an example of the latter, RCU's dyntick-idle nesting counter - allows 54 bits for interrupt nesting level (this counter - is 64 bits even on a 32-bit system). - Overflowing this counter requires 2<sup>54</sup> - half-interrupts on a given CPU without that CPU ever going idle. - If a half-interrupt happened every microsecond, it would take - 570 years of runtime to overflow this counter, which is currently - believed to be an acceptably long time. -<li> Linux systems can have thousands of CPUs running a single - Linux kernel in a single shared-memory environment. - RCU must therefore pay close attention to high-end scalability. -</ol> - -<p> -This last parallelism fact of life means that RCU must pay special -attention to the preceding facts of life. -The idea that Linux might scale to systems with thousands of CPUs would -have been met with some skepticism in the 1990s, but these requirements -would have otherwise have been unsurprising, even in the early 1990s. - -<h2><a name="Quality-of-Implementation Requirements">Quality-of-Implementation Requirements</a></h2> - -<p> -These sections list quality-of-implementation requirements. -Although an RCU implementation that ignores these requirements could -still be used, it would likely be subject to limitations that would -make it inappropriate for industrial-strength production use. -Classes of quality-of-implementation requirements are as follows: - -<ol> -<li> <a href="#Specialization">Specialization</a> -<li> <a href="#Performance and Scalability">Performance and Scalability</a> -<li> <a href="#Composability">Composability</a> -<li> <a href="#Corner Cases">Corner Cases</a> -</ol> - -<p> -These classes is covered in the following sections. - -<h3><a name="Specialization">Specialization</a></h3> - -<p> -RCU is and always has been intended primarily for read-mostly situations, as -illustrated by the following figure. -This means that RCU's read-side primitives are optimized, often at the -expense of its update-side primitives. - -<p><img src="RCUApplicability.svg" alt="RCUApplicability.svg" width="70%"></p> - -<p> -This focus on read-mostly situations means that RCU must interoperate -with other synchronization primitives. -For example, the <tt>add_gp()</tt> and <tt>remove_gp_synchronous()</tt> -examples discussed earlier use RCU to protect readers and locking to -coordinate updaters. -However, the need extends much farther, requiring that a variety of -synchronization primitives be legal within RCU read-side critical sections, -including spinlocks, sequence locks, atomic operations, reference -counters, and memory barriers. - -<p>@@QQ@@ -What about sleeping locks? -<p>@@QQA@@ -These are forbidden within Linux-kernel RCU read-side critical sections -because it is not legal to place a quiescent state (in this case, -voluntary context switch) within an RCU read-side critical section. -However, sleeping locks may be used within userspace RCU read-side critical -sections, and also within Linux-kernel sleepable RCU -<a href="#Sleepable RCU">(SRCU)</a> -read-side critical sections. -In addition, the -rt patchset turns spinlocks into a sleeping locks so -that the corresponding critical sections can be preempted, which -also means that these sleeplockified spinlocks (but not other sleeping locks!) -may be acquire within -rt-Linux-kernel RCU read-side critical sections. - -<p> -Note that it <i>is</i> legal for a normal RCU read-side critical section -to conditionally acquire a sleeping locks (as in <tt>mutex_trylock()</tt>), -but only as long as it does not loop indefinitely attempting to -conditionally acquire that sleeping locks. -The key point is that things like <tt>mutex_trylock()</tt> -either return with the mutex held, or return an error indication if -the mutex was not immediately available. -Either way, <tt>mutex_trylock()</tt> returns immediately without sleeping. -<p>@@QQE@@ - -<p> -It often comes as a surprise that many algorithms do not require a -consistent view of data, but many can function in that mode, -with network routing being the poster child. -Internet routing algorithms take significant time to propagate -updates, so that by the time an update arrives at a given system, -that system has been sending network traffic the wrong way for -a considerable length of time. -Having a few threads continue to send traffic the wrong way for a -few more milliseconds is clearly not a problem: In the worst case, -TCP retransmissions will eventually get the data where it needs to go. -In general, when tracking the state of the universe outside of the -computer, some level of inconsistency must be tolerated due to -speed-of-light delays if nothing else. - -<p> -Furthermore, uncertainty about external state is inherent in many cases. -For example, a pair of veternarians might use heartbeat to determine -whether or not a given cat was alive. -But how long should they wait after the last heartbeat to decide that -the cat is in fact dead? -Waiting less than 400 milliseconds makes no sense because this would -mean that a relaxed cat would be considered to cycle between death -and life more than 100 times per minute. -Moreover, just as with human beings, a cat's heart might stop for -some period of time, so the exact wait period is a judgment call. -One of our pair of veternarians might wait 30 seconds before pronouncing -the cat dead, while the other might insist on waiting a full minute. -The two veternarians would then disagree on the state of the cat during -the final 30 seconds of the minute following the last heartbeat, as -fancifully illustrated below: - -<p><img src="2013-08-is-it-dead.png" alt="2013-08-is-it-dead.png" width="431"></p> - -<p> -Interestingly enough, this same situation applies to hardware. -When push comes to shove, how do we tell whether or not some -external server has failed? -We send messages to it periodically, and declare it failed if we -don't receive a response within a given period of time. -Policy decisions can usually tolerate short -periods of inconsistency. -The policy was decided some time ago, and is only now being put into -effect, so a few milliseconds of delay is normally inconsequential. - -<p> -However, there are algorithms that absolutely must see consistent data. -For example, the translation between a user-level SystemV semaphore -ID to the corresponding in-kernel data structure is protected by RCU, -but it is absolutely forbidden to update a semaphore that has just been -removed. -In the Linux kernel, this need for consistency is accommodated by acquiring -spinlocks located in the in-kernel data structure from within -the RCU read-side critical section, and this is indicated by the -green box in the figure above. -Many other techniques may be used, and are in fact used within the -Linux kernel. - -<p> -In short, RCU is not required to maintain consistency, and other -mechanisms may be used in concert with RCU when consistency is required. -RCU's specialization allows it to do its job extremely well, and its -ability to interoperate with other synchronization mechanisms allows -the right mix of synchronization tools to be used for a given job. - -<h3><a name="Performance and Scalability">Performance and Scalability</a></h3> - -<p> -Energy efficiency is a critical component of performance today, -and Linux-kernel RCU implementations must therefore avoid unnecessarily -awakening idle CPUs. -I cannot claim that this requirement was premeditated. -In fact, I learned of it during a telephone conversation in which I -was given “frank and open” feedback on the importance -of energy efficiency in battery-powered systems and on specific -energy-efficiency shortcomings of the Linux-kernel RCU implementation. -In my experience, the battery-powered embedded community will consider -any unnecessary wakeups to be extremely unfriendly acts. -So much so that mere Linux-kernel-mailing-list posts are -insufficient to vent their ire. - -<p> -Memory consumption is not particularly important for in most -situations, and has become decreasingly -so as memory sizes have expanded and memory -costs have plummeted. -However, as I learned from Matt Mackall's -<a href="http://elinux.org/Linux_Tiny-FAQ">bloatwatch</a> -efforts, memory footprint is critically important on single-CPU systems with -non-preemptible (<tt>CONFIG_PREEMPT=n</tt>) kernels, and thus -<a href="https://lkml.kernel.org/g/20090113221724.GA15307@linux.vnet.ibm.com">tiny RCU</a> -was born. -Josh Triplett has since taken over the small-memory banner with his -<a href="https://tiny.wiki.kernel.org/">Linux kernel tinification</a> -project, which resulted in -<a href="#Sleepable RCU">SRCU</a> -becoming optional for those kernels not needing it. - -<p> -The remaining performance requirements are, for the most part, -unsurprising. -For example, in keeping with RCU's read-side specialization, -<tt>rcu_dereference()</tt> should have negligible overhead (for -example, suppression of a few minor compiler optimizations). -Similarly, in non-preemptible environments, <tt>rcu_read_lock()</tt> and -<tt>rcu_read_unlock()</tt> should have exactly zero overhead. - -<p> -In preemptible environments, in the case where the RCU read-side -critical section was not preempted (as will be the case for the -highest-priority real-time process), <tt>rcu_read_lock()</tt> and -<tt>rcu_read_unlock()</tt> should have minimal overhead. -In particular, they should not contain atomic read-modify-write -operations, memory-barrier instructions, preemption disabling, -interrupt disabling, or backwards branches. -However, in the case where the RCU read-side critical section was preempted, -<tt>rcu_read_unlock()</tt> may acquire spinlocks and disable interrupts. -This is why it is better to nest an RCU read-side critical section -within a preempt-disable region than vice versa, at least in cases -where that critical section is short enough to avoid unduly degrading -real-time latencies. - -<p> -The <tt>synchronize_rcu()</tt> grace-period-wait primitive is -optimized for throughput. -It may therefore incur several milliseconds of latency in addition to -the duration of the longest RCU read-side critical section. -On the other hand, multiple concurrent invocations of -<tt>synchronize_rcu()</tt> are required to use batching optimizations -so that they can be satisfied by a single underlying grace-period-wait -operation. -For example, in the Linux kernel, it is not unusual for a single -grace-period-wait operation to serve more than -<a href="https://www.usenix.org/conference/2004-usenix-annual-technical-conference/making-rcu-safe-deep-sub-millisecond-response">1,000 separate invocations</a> -of <tt>synchronize_rcu()</tt>, thus amortizing the per-invocation -overhead down to nearly zero. -However, the grace-period optimization is also required to avoid -measurable degradation of real-time scheduling and interrupt latencies. - -<p> -In some cases, the multi-millisecond <tt>synchronize_rcu()</tt> -latencies are unacceptable. -In these cases, <tt>synchronize_rcu_expedited()</tt> may be used -instead, reducing the grace-period latency down to a few tens of -microseconds on small systems, at least in cases where the RCU read-side -critical sections are short. -There are currently no special latency requirements for -<tt>synchronize_rcu_expedited()</tt> on large systems, but, -consistent with the empirical nature of the RCU specification, -that is subject to change. -However, there most definitely are scalability requirements: -A storm of <tt>synchronize_rcu_expedited()</tt> invocations on 4096 -CPUs should at least make reasonable forward progress. -In return for its shorter latencies, <tt>synchronize_rcu_expedited()</tt> -is permitted to impose modest degradation of real-time latency -on non-idle online CPUs. -That said, it will likely be necessary to take further steps to reduce this -degradation, hopefully to roughly that of a scheduling-clock interrupt. - -<p> -There are a number of situations where even -<tt>synchronize_rcu_expedited()</tt>'s reduced grace-period -latency is unacceptable. -In these situations, the asynchronous <tt>call_rcu()</tt> can be -used in place of <tt>synchronize_rcu()</tt> as follows: - -<blockquote> -<pre> - 1 struct foo { - 2 int a; - 3 int b; - 4 struct rcu_head rh; - 5 }; - 6 - 7 static void remove_gp_cb(struct rcu_head *rhp) - 8 { - 9 struct foo *p = container_of(rhp, struct foo, rh); -10 -11 kfree(p); -12 } -13 -14 bool remove_gp_asynchronous(void) -15 { -16 struct foo *p; -17 -18 spin_lock(&gp_lock); -19 p = rcu_dereference(gp); -20 if (!p) { -21 spin_unlock(&gp_lock); -22 return false; -23 } -24 rcu_assign_pointer(gp, NULL); -25 call_rcu(&p->rh, remove_gp_cb); -26 spin_unlock(&gp_lock); -27 return true; -28 } -</pre> -</blockquote> - -<p> -A definition of <tt>struct foo</tt> is finally needed, and appears -on lines 1-5. -The function <tt>remove_gp_cb()</tt> is passed to <tt>call_rcu()</tt> -on line 25, and will be invoked after the end of a subsequent -grace period. -This gets the same effect as <tt>remove_gp_synchronous()</tt>, -but without forcing the updater to wait for a grace period to elapse. -The <tt>call_rcu()</tt> function may be used in a number of -situations where neither <tt>synchronize_rcu()</tt> nor -<tt>synchronize_rcu_expedited()</tt> would be legal, -including within preempt-disable code, <tt>local_bh_disable()</tt> code, -interrupt-disable code, and interrupt handlers. -However, even <tt>call_rcu()</tt> is illegal within NMI handlers. -The callback function (<tt>remove_gp_cb()</tt> in this case) will be -executed within softirq (software interrupt) environment within the -Linux kernel, -either within a real softirq handler or under the protection -of <tt>local_bh_disable()</tt>. -In both the Linux kernel and in userspace, it is bad practice to -write an RCU callback function that takes too long. -Long-running operations should be relegated to separate threads or -(in the Linux kernel) workqueues. - -<p>@@QQ@@ -Why does line 19 use <tt>rcu_access_pointer()</tt>? -After all, <tt>call_rcu()</tt> on line 25 stores into the -structure, which would interact badly with concurrent insertions. -Doesn't this mean that <tt>rcu_dereference()</tt> is required? -<p>@@QQA@@ -Presumably the <tt>->gp_lock</tt> acquired on line 18 excludes -any changes, including any insertions that <tt>rcu_dereference()</tt> -would protect against. -Therefore, any insertions will be delayed until after <tt>->gp_lock</tt> -is released on line 25, which in turn means that -<tt>rcu_access_pointer()</tt> suffices. -<p>@@QQE@@ - -<p> -However, all that <tt>remove_gp_cb()</tt> is doing is -invoking <tt>kfree()</tt> on the data element. -This is a common idiom, and is supported by <tt>kfree_rcu()</tt>, -which allows “fire and forget” operation as shown below: - -<blockquote> -<pre> - 1 struct foo { - 2 int a; - 3 int b; - 4 struct rcu_head rh; - 5 }; - 6 - 7 bool remove_gp_faf(void) - 8 { - 9 struct foo *p; -10 -11 spin_lock(&gp_lock); -12 p = rcu_dereference(gp); -13 if (!p) { -14 spin_unlock(&gp_lock); -15 return false; -16 } -17 rcu_assign_pointer(gp, NULL); -18 kfree_rcu(p, rh); -19 spin_unlock(&gp_lock); -20 return true; -21 } -</pre> -</blockquote> - -<p> -Note that <tt>remove_gp_faf()</tt> simply invokes -<tt>kfree_rcu()</tt> and proceeds, without any need to pay any -further attention to the subsequent grace period and <tt>kfree()</tt>. -It is permissible to invoke <tt>kfree_rcu()</tt> from the same -environments as for <tt>call_rcu()</tt>. -Interestingly enough, DYNIX/ptx had the equivalents of -<tt>call_rcu()</tt> and <tt>kfree_rcu()</tt>, but not -<tt>synchronize_rcu()</tt>. -This was due to the fact that RCU was not heavily used within DYNIX/ptx, -so the very few places that needed something like -<tt>synchronize_rcu()</tt> simply open-coded it. - -<p>@@QQ@@ -Earlier it was claimed that <tt>call_rcu()</tt> and -<tt>kfree_rcu()</tt> allowed updaters to avoid being blocked -by readers. -But how can that be correct, given that the invocation of the callback -and the freeing of the memory (respectively) must still wait for -a grace period to elapse? -<p>@@QQA@@ -We could define things this way, but keep in mind that this sort of -definition would say that updates in garbage-collected languages -cannot complete until the next time the garbage collector runs, -which does not seem at all reasonable. -The key point is that in most cases, an updater using either -<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt> can proceed to the -next update as soon as it has invoked <tt>call_rcu()</tt> or -<tt>kfree_rcu()</tt>, without having to wait for a subsequent -grace period. -<p>@@QQE@@ - -<p> -But what if the updater must wait for the completion of code to be -executed after the end of the grace period, but has other tasks -that can be carried out in the meantime? -The polling-style <tt>get_state_synchronize_rcu()</tt> and -<tt>cond_synchronize_rcu()</tt> functions may be used for this -purpose, as shown below: - -<blockquote> -<pre> - 1 bool remove_gp_poll(void) - 2 { - 3 struct foo *p; - 4 unsigned long s; - 5 - 6 spin_lock(&gp_lock); - 7 p = rcu_access_pointer(gp); - 8 if (!p) { - 9 spin_unlock(&gp_lock); -10 return false; -11 } -12 rcu_assign_pointer(gp, NULL); -13 spin_unlock(&gp_lock); -14 s = get_state_synchronize_rcu(); -15 do_something_while_waiting(); -16 cond_synchronize_rcu(s); -17 kfree(p); -18 return true; -19 } -</pre> -</blockquote> - -<p> -On line 14, <tt>get_state_synchronize_rcu()</tt> obtains a -“cookie” from RCU, -then line 15 carries out other tasks, -and finally, line 16 returns immediately if a grace period has -elapsed in the meantime, but otherwise waits as required. -The need for <tt>get_state_synchronize_rcu</tt> and -<tt>cond_synchronize_rcu()</tt> has appeared quite recently, -so it is too early to tell whether they will stand the test of time. - -<p> -RCU thus provides a range of tools to allow updaters to strike the -required tradeoff between latency, flexibility and CPU overhead. - -<h3><a name="Composability">Composability</a></h3> - -<p> -Composability has received much attention in recent years, perhaps in part -due to the collision of multicore hardware with object-oriented techniques -designed in single-threaded environments for single-threaded use. -And in theory, RCU read-side critical sections may be composed, and in -fact may be nested arbitrarily deeply. -In practice, as with all real-world implementations of composable -constructs, there are limitations. - -<p> -Implementations of RCU for which <tt>rcu_read_lock()</tt> -and <tt>rcu_read_unlock()</tt> generate no code, such as -Linux-kernel RCU when <tt>CONFIG_PREEMPT=n</tt>, can be -nested arbitrarily deeply. -After all, there is no overhead. -Except that if all these instances of <tt>rcu_read_lock()</tt> -and <tt>rcu_read_unlock()</tt> are visible to the compiler, -compilation will eventually fail due to exhausting memory, -mass storage, or user patience, whichever comes first. -If the nesting is not visible to the compiler, as is the case with -mutually recursive functions each in its own translation unit, -stack overflow will result. -If the nesting takes the form of loops, either the control variable -will overflow or (in the Linux kernel) you will get an RCU CPU stall warning. -Nevertheless, this class of RCU implementations is one -of the most composable constructs in existence. - -<p> -RCU implementations that explicitly track nesting depth -are limited by the nesting-depth counter. -For example, the Linux kernel's preemptible RCU limits nesting to -<tt>INT_MAX</tt>. -This should suffice for almost all practical purposes. -That said, a consecutive pair of RCU read-side critical sections -between which there is an operation that waits for a grace period -cannot be enclosed in another RCU read-side critical section. -This is because it is not legal to wait for a grace period within -an RCU read-side critical section: To do so would result either -in deadlock or -in RCU implicitly splitting the enclosing RCU read-side critical -section, neither of which is conducive to a long-lived and prosperous -kernel. - -<p> -It is worth noting that RCU is not alone in limiting composability. -For example, many transactional-memory implementations prohibit -composing a pair of transactions separated by an irrevocable -operation (for example, a network receive operation). -For another example, lock-based critical sections can be composed -surprisingly freely, but only if deadlock is avoided. - -<p> -In short, although RCU read-side critical sections are highly composable, -care is required in some situations, just as is the case for any other -composable synchronization mechanism. - -<h3><a name="Corner Cases">Corner Cases</a></h3> - -<p> -A given RCU workload might have an endless and intense stream of -RCU read-side critical sections, perhaps even so intense that there -was never a point in time during which there was not at least one -RCU read-side critical section in flight. -RCU cannot allow this situation to block grace periods: As long as -all the RCU read-side critical sections are finite, grace periods -must also be finite. - -<p> -That said, preemptible RCU implementations could potentially result -in RCU read-side critical sections being preempted for long durations, -which has the effect of creating a long-duration RCU read-side -critical section. -This situation can arise only in heavily loaded systems, but systems using -real-time priorities are of course more vulnerable. -Therefore, RCU priority boosting is provided to help deal with this -case. -That said, the exact requirements on RCU priority boosting will likely -evolve as more experience accumulates. - -<p> -Other workloads might have very high update rates. -Although one can argue that such workloads should instead use -something other than RCU, the fact remains that RCU must -handle such workloads gracefully. -This requirement is another factor driving batching of grace periods, -but it is also the driving force behind the checks for large numbers -of queued RCU callbacks in the <tt>call_rcu()</tt> code path. -Finally, high update rates should not delay RCU read-side critical -sections, although some read-side delays can occur when using -<tt>synchronize_rcu_expedited()</tt>, courtesy of this function's use -of <tt>try_stop_cpus()</tt>. -(In the future, <tt>synchronize_rcu_expedited()</tt> will be -converted to use lighter-weight inter-processor interrupts (IPIs), -but this will still disturb readers, though to a much smaller degree.) - -<p> -Although all three of these corner cases were understood in the early -1990s, a simple user-level test consisting of <tt>close(open(path))</tt> -in a tight loop -in the early 2000s suddenly provided a much deeper appreciation of the -high-update-rate corner case. -This test also motivated addition of some RCU code to react to high update -rates, for example, if a given CPU finds itself with more than 10,000 -RCU callbacks queued, it will cause RCU to take evasive action by -more aggressively starting grace periods and more aggressively forcing -completion of grace-period processing. -This evasive action causes the grace period to complete more quickly, -but at the cost of restricting RCU's batching optimizations, thus -increasing the CPU overhead incurred by that grace period. - -<h2><a name="Software-Engineering Requirements"> -Software-Engineering Requirements</a></h2> - -<p> -Between Murphy's Law and “To err is human”, it is necessary to -guard against mishaps and misuse: - -<ol> -<li> It is all too easy to forget to use <tt>rcu_read_lock()</tt> - everywhere that it is needed, so kernels built with - <tt>CONFIG_PROVE_RCU=y</tt> will spat if - <tt>rcu_dereference()</tt> is used outside of an - RCU read-side critical section. - Update-side code can use <tt>rcu_dereference_protected()</tt>, - which takes a - <a href="https://lwn.net/Articles/371986/">lockdep expression</a> - to indicate what is providing the protection. - If the indicated protection is not provided, a lockdep splat - is emitted. - - <p> - Code shared between readers and updaters can use - <tt>rcu_dereference_check()</tt>, which also takes a - lockdep expression, and emits a lockdep splat if neither - <tt>rcu_read_lock()</tt> nor the indicated protection - is in place. - In addition, <tt>rcu_dereference_raw()</tt> is used in those - (hopefully rare) cases where the required protection cannot - be easily described. - Finally, <tt>rcu_read_lock_held()</tt> is provided to - allow a function to verify that it has been invoked within - an RCU read-side critical section. - I was made aware of this set of requirements shortly after Thomas - Gleixner audited a number of RCU uses. -<li> A given function might wish to check for RCU-related preconditions - upon entry, before using any other RCU API. - The <tt>rcu_lockdep_assert()</tt> does this job, - asserting the expression in kernels having lockdep enabled - and doing nothing otherwise. -<li> It is also easy to forget to use <tt>rcu_assign_pointer()</tt> - and <tt>rcu_dereference()</tt>, perhaps (incorrectly) - substituting a simple assignment. - To catch this sort of error, a given RCU-protected pointer may be - tagged with <tt>__rcu</tt>, after which running sparse - with <tt>CONFIG_SPARSE_RCU_POINTER=y</tt> will complain - about simple-assignment accesses to that pointer. - Arnd Bergmann made me aware of this requirement, and also - supplied the needed - <a href="https://lwn.net/Articles/376011/">patch series</a>. -<li> Kernels built with <tt>CONFIG_DEBUG_OBJECTS_RCU_HEAD=y</tt> - will splat if a data element is passed to <tt>call_rcu()</tt> - twice in a row, without a grace period in between. - (This error is similar to a double free.) - The corresponding <tt>rcu_head</tt> structures that are - dynamically allocated are automatically tracked, but - <tt>rcu_head</tt> structures allocated on the stack - must be initialized with <tt>init_rcu_head_on_stack()</tt> - and cleaned up with <tt>destroy_rcu_head_on_stack()</tt>. - Similarly, statically allocated non-stack <tt>rcu_head</tt> - structures must be initialized with <tt>init_rcu_head()</tt> - and cleaned up with <tt>destroy_rcu_head()</tt>. - Mathieu Desnoyers made me aware of this requirement, and also - supplied the needed - <a href="https://lkml.kernel.org/g/20100319013024.GA28456@Krystal">patch</a>. -<li> An infinite loop in an RCU read-side critical section will - eventually trigger an RCU CPU stall warning splat, with - the duration of “eventually” being controlled by the - <tt>RCU_CPU_STALL_TIMEOUT</tt> <tt>Kconfig</tt> option, or, - alternatively, by the - <tt>rcupdate.rcu_cpu_stall_timeout</tt> boot/sysfs - parameter. - However, RCU is not obligated to produce this splat - unless there is a grace period waiting on that particular - RCU read-side critical section. - <p> - Some extreme workloads might intentionally delay - RCU grace periods, and systems running those workloads can - be booted with <tt>rcupdate.rcu_cpu_stall_suppress</tt> - to suppress the splats. - This kernel parameter may also be set via <tt>sysfs</tt>. - Furthermore, RCU CPU stall warnings are counter-productive - during sysrq dumps and during panics. - RCU therefore supplies the <tt>rcu_sysrq_start()</tt> and - <tt>rcu_sysrq_end()</tt> API members to be called before - and after long sysrq dumps. - RCU also supplies the <tt>rcu_panic()</tt> notifier that is - automatically invoked at the beginning of a panic to suppress - further RCU CPU stall warnings. - - <p> - This requirement made itself known in the early 1990s, pretty - much the first time that it was necessary to debug a CPU stall. - That said, the initial implementation in DYNIX/ptx was quite - generic in comparison with that of Linux. -<li> Although it would be very good to detect pointers leaking out - of RCU read-side critical sections, there is currently no - good way of doing this. - One complication is the need to distinguish between pointers - leaking and pointers that have been handed off from RCU to - some other synchronization mechanism, for example, reference - counting. -<li> In kernels built with <tt>CONFIG_RCU_TRACE=y</tt>, RCU-related - information is provided via both debugfs and event tracing. -<li> Open-coded use of <tt>rcu_assign_pointer()</tt> and - <tt>rcu_dereference()</tt> to create typical linked - data structures can be surprisingly error-prone. - Therefore, RCU-protected - <a href="https://lwn.net/Articles/609973/#RCU List APIs">linked lists</a> - and, more recently, RCU-protected - <a href="https://lwn.net/Articles/612100/">hash tables</a> - are available. - Many other special-purpose RCU-protected data structures are - available in the Linux kernel and the userspace RCU library. -<li> Some linked structures are created at compile time, but still - require <tt>__rcu</tt> checking. - The <tt>RCU_POINTER_INITIALIZER()</tt> macro serves this - purpose. -<li> It is not necessary to use <tt>rcu_assign_pointer()</tt> - when creating linked structures that are to be published via - a single external pointer. - The <tt>RCU_INIT_POINTER()</tt> macro is provided for - this task and also for assigning <tt>NULL</tt> pointers - at runtime. -</ol> - -<p> -This not a hard-and-fast list: RCU's diagnostic capabilities will -continue to be guided by the number and type of usage bugs found -in real-world RCU usage. - -<h2><a name="Linux Kernel Complications">Linux Kernel Complications</a></h2> - -<p> -The Linux kernel provides an interesting environment for all kinds of -software, including RCU. -Some of the relevant points of interest are as follows: - -<ol> -<li> <a href="#Configuration">Configuration</a>. -<li> <a href="#Firmware Interface">Firmware Interface</a>. -<li> <a href="#Early Boot">Early Boot</a>. -<li> <a href="#Interrupts and NMIs"> - Interrupts and non-maskable interrupts (NMIs)</a>. -<li> <a href="#Loadable Modules">Loadable Modules</a>. -<li> <a href="#Hotplug CPU">Hotplug CPU</a>. -<li> <a href="#Scheduler and RCU">Scheduler and RCU</a>. -<li> <a href="#Tracing and RCU">Tracing and RCU</a>. -<li> <a href="#Energy Efficiency">Energy Efficiency</a>. -<li> <a href="#Memory Efficiency">Memory Efficiency</a>. -<li> <a href="#Performance, Scalability, Response Time, and Reliability"> - Performance, Scalability, Response Time, and Reliability</a>. -</ol> - -<p> -This list is probably incomplete, but it does give a feel for the -most notable Linux-kernel complications. -Each of the following sections covers one of the above topics. - -<h3><a name="Configuration">Configuration</a></h3> - -<p> -RCU's goal is automatic configuration, so that almost nobody -needs to worry about RCU's <tt>Kconfig</tt> options. -And for almost all users, RCU does in fact work well -“out of the box.” - -<p> -However, there are specialized use cases that are handled by -kernel boot parameters and <tt>Kconfig</tt> options. -Unfortunately, the <tt>Kconfig</tt> system will explicitly ask users -about new <tt>Kconfig</tt> options, which requires almost all of them -be hidden behind a <tt>CONFIG_RCU_EXPERT</tt> <tt>Kconfig</tt> option. - -<p> -This all should be quite obvious, but the fact remains that -Linus Torvalds recently had to -<a href="https://lkml.kernel.org/g/CA+55aFy4wcCwaL4okTs8wXhGZ5h-ibecy_Meg9C4MNQrUnwMcg@mail.gmail.com">remind</a> -me of this requirement. - -<h3><a name="Firmware Interface">Firmware Interface</a></h3> - -<p> -In many cases, kernel obtains information about the system from the -firmware, and sometimes things are lost in translation. -Or the translation is accurate, but the original message is bogus. - -<p> -For example, some systems' firmware overreports the number of CPUs, -sometimes by a large factor. -If RCU naively believed the firmware, as it used to do, -it would create too many per-CPU kthreads. -Although the resulting system will still run correctly, the extra -kthreads needlessly consume memory and can cause confusion -when they show up in <tt>ps</tt> listings. - -<p> -RCU must therefore wait for a given CPU to actually come online before -it can allow itself to believe that the CPU actually exists. -The resulting “ghost CPUs” (which are never going to -come online) cause a number of -<a href="https://paulmck.livejournal.com/37494.html">interesting complications</a>. - -<h3><a name="Early Boot">Early Boot</a></h3> - -<p> -The Linux kernel's boot sequence is an interesting process, -and RCU is used early, even before <tt>rcu_init()</tt> -is invoked. -In fact, a number of RCU's primitives can be used as soon as the -initial task's <tt>task_struct</tt> is available and the -boot CPU's per-CPU variables are set up. -The read-side primitives (<tt>rcu_read_lock()</tt>, -<tt>rcu_read_unlock()</tt>, <tt>rcu_dereference()</tt>, -and <tt>rcu_access_pointer()</tt>) will operate normally very early on, -as will <tt>rcu_assign_pointer()</tt>. - -<p> -Although <tt>call_rcu()</tt> may be invoked at any -time during boot, callbacks are not guaranteed to be invoked until after -the scheduler is fully up and running. -This delay in callback invocation is due to the fact that RCU does not -invoke callbacks until it is fully initialized, and this full initialization -cannot occur until after the scheduler has initialized itself to the -point where RCU can spawn and run its kthreads. -In theory, it would be possible to invoke callbacks earlier, -however, this is not a panacea because there would be severe restrictions -on what operations those callbacks could invoke. - -<p> -Perhaps surprisingly, <tt>synchronize_rcu()</tt>, -<a href="#Bottom-Half Flavor"><tt>synchronize_rcu_bh()</tt></a> -(<a href="#Bottom-Half Flavor">discussed below</a>), -and -<a href="#Sched Flavor"><tt>synchronize_sched()</tt></a> -will all operate normally -during very early boot, the reason being that there is only one CPU -and preemption is disabled. -This means that the call <tt>synchronize_rcu()</tt> (or friends) -itself is a quiescent -state and thus a grace period, so the early-boot implementation can -be a no-op. - -<p> -Both <tt>synchronize_rcu_bh()</tt> and <tt>synchronize_sched()</tt> -continue to operate normally through the remainder of boot, courtesy -of the fact that preemption is disabled across their RCU read-side -critical sections and also courtesy of the fact that there is still -only one CPU. -However, once the scheduler starts initializing, preemption is enabled. -There is still only a single CPU, but the fact that preemption is enabled -means that the no-op implementation of <tt>synchronize_rcu()</tt> no -longer works in <tt>CONFIG_PREEMPT=y</tt> kernels. -Therefore, as soon as the scheduler starts initializing, the early-boot -fastpath is disabled. -This means that <tt>synchronize_rcu()</tt> switches to its runtime -mode of operation where it posts callbacks, which in turn means that -any call to <tt>synchronize_rcu()</tt> will block until the corresponding -callback is invoked. -Unfortunately, the callback cannot be invoked until RCU's runtime -grace-period machinery is up and running, which cannot happen until -the scheduler has initialized itself sufficiently to allow RCU's -kthreads to be spawned. -Therefore, invoking <tt>synchronize_rcu()</tt> during scheduler -initialization can result in deadlock. - -<p>@@QQ@@ -So what happens with <tt>synchronize_rcu()</tt> during -scheduler initialization for <tt>CONFIG_PREEMPT=n</tt> -kernels? -<p>@@QQA@@ -In <tt>CONFIG_PREEMPT=n</tt> kernel, <tt>synchronize_rcu()</tt> -maps directly to <tt>synchronize_sched()</tt>. -Therefore, <tt>synchronize_rcu()</tt> works normally throughout -boot in <tt>CONFIG_PREEMPT=n</tt> kernels. -However, your code must also work in <tt>CONFIG_PREEMPT=y</tt> kernels, -so it is still necessary to avoid invoking <tt>synchronize_rcu()</tt> -during scheduler initialization. -<p>@@QQE@@ - -<p> -I learned of these boot-time requirements as a result of a series of -system hangs. - -<h3><a name="Interrupts and NMIs">Interrupts and NMIs</a></h3> - -<p> -The Linux kernel has interrupts, and RCU read-side critical sections are -legal within interrupt handlers and within interrupt-disabled regions -of code, as are invocations of <tt>call_rcu()</tt>. - -<p> -Some Linux-kernel architectures can enter an interrupt handler from -non-idle process context, and then just never leave it, instead stealthily -transitioning back to process context. -This trick is sometimes used to invoke system calls from inside the kernel. -These “half-interrupts” mean that RCU has to be very careful -about how it counts interrupt nesting levels. -I learned of this requirement the hard way during a rewrite -of RCU's dyntick-idle code. - -<p> -The Linux kernel has non-maskable interrupts (NMIs), and -RCU read-side critical sections are legal within NMI handlers. -Thankfully, RCU update-side primitives, including -<tt>call_rcu()</tt>, are prohibited within NMI handlers. - -<p> -The name notwithstanding, some Linux-kernel architectures -can have nested NMIs, which RCU must handle correctly. -Andy Lutomirski -<a href="https://lkml.kernel.org/g/CALCETrXLq1y7e_dKFPgou-FKHB6Pu-r8+t-6Ds+8=va7anBWDA@mail.gmail.com">surprised me</a> -with this requirement; -he also kindly surprised me with -<a href="https://lkml.kernel.org/g/CALCETrXSY9JpW3uE6H8WYk81sg56qasA2aqmjMPsq5dOtzso=g@mail.gmail.com">an algorithm</a> -that meets this requirement. - -<h3><a name="Loadable Modules">Loadable Modules</a></h3> - -<p> -The Linux kernel has loadable modules, and these modules can -also be unloaded. -After a given module has been unloaded, any attempt to call -one of its functions results in a segmentation fault. -The module-unload functions must therefore cancel any -delayed calls to loadable-module functions, for example, -any outstanding <tt>mod_timer()</tt> must be dealt with -via <tt>del_timer_sync()</tt> or similar. - -<p> -Unfortunately, there is no way to cancel an RCU callback; -once you invoke <tt>call_rcu()</tt>, the callback function is -going to eventually be invoked, unless the system goes down first. -Because it is normally considered socially irresponsible to crash the system -in response to a module unload request, we need some other way -to deal with in-flight RCU callbacks. - -<p> -RCU therefore provides -<tt><a href="https://lwn.net/Articles/217484/">rcu_barrier()</a></tt>, -which waits until all in-flight RCU callbacks have been invoked. -If a module uses <tt>call_rcu()</tt>, its exit function should therefore -prevent any future invocation of <tt>call_rcu()</tt>, then invoke -<tt>rcu_barrier()</tt>. -In theory, the underlying module-unload code could invoke -<tt>rcu_barrier()</tt> unconditionally, but in practice this would -incur unacceptable latencies. - -<p> -Nikita Danilov noted this requirement for an analogous filesystem-unmount -situation, and Dipankar Sarma incorporated <tt>rcu_barrier()</tt> into RCU. -The need for <tt>rcu_barrier()</tt> for module unloading became -apparent later. - -<h3><a name="Hotplug CPU">Hotplug CPU</a></h3> - -<p> -The Linux kernel supports CPU hotplug, which means that CPUs -can come and go. -It is of course illegal to use any RCU API member from an offline CPU. -This requirement was present from day one in DYNIX/ptx, but -on the other hand, the Linux kernel's CPU-hotplug implementation -is “interesting.” - -<p> -The Linux-kernel CPU-hotplug implementation has notifiers that -are used to allow the various kernel subsystems (including RCU) -to respond appropriately to a given CPU-hotplug operation. -Most RCU operations may be invoked from CPU-hotplug notifiers, -including even normal synchronous grace-period operations -such as <tt>synchronize_rcu()</tt>. -However, expedited grace-period operations such as -<tt>synchronize_rcu_expedited()</tt> are not supported, -due to the fact that current implementations block CPU-hotplug -operations, which could result in deadlock. - -<p> -In addition, all-callback-wait operations such as -<tt>rcu_barrier()</tt> are also not supported, due to the -fact that there are phases of CPU-hotplug operations where -the outgoing CPU's callbacks will not be invoked until after -the CPU-hotplug operation ends, which could also result in deadlock. - -<h3><a name="Scheduler and RCU">Scheduler and RCU</a></h3> - -<p> -RCU depends on the scheduler, and the scheduler uses RCU to -protect some of its data structures. -This means the scheduler is forbidden from acquiring -the runqueue locks and the priority-inheritance locks -in the middle of an outermost RCU read-side critical section unless either -(1) it releases them before exiting that same -RCU read-side critical section, or -(2) interrupts are disabled across -that entire RCU read-side critical section. -This same prohibition also applies (recursively!) to any lock that is acquired -while holding any lock to which this prohibition applies. -Adhering to this rule prevents preemptible RCU from invoking -<tt>rcu_read_unlock_special()</tt> while either runqueue or -priority-inheritance locks are held, thus avoiding deadlock. - -<p> -Prior to v4.4, it was only necessary to disable preemption across -RCU read-side critical sections that acquired scheduler locks. -In v4.4, expedited grace periods started using IPIs, and these -IPIs could force a <tt>rcu_read_unlock()</tt> to take the slowpath. -Therefore, this expedited-grace-period change required disabling of -interrupts, not just preemption. - -<p> -For RCU's part, the preemptible-RCU <tt>rcu_read_unlock()</tt> -implementation must be written carefully to avoid similar deadlocks. -In particular, <tt>rcu_read_unlock()</tt> must tolerate an -interrupt where the interrupt handler invokes both -<tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>. -This possibility requires <tt>rcu_read_unlock()</tt> to use -negative nesting levels to avoid destructive recursion via -interrupt handler's use of RCU. - -<p> -This pair of mutual scheduler-RCU requirements came as a -<a href="https://lwn.net/Articles/453002/">complete surprise</a>. - -<p> -As noted above, RCU makes use of kthreads, and it is necessary to -avoid excessive CPU-time accumulation by these kthreads. -This requirement was no surprise, but RCU's violation of it -when running context-switch-heavy workloads when built with -<tt>CONFIG_NO_HZ_FULL=y</tt> -<a href="http://www.rdrop.com/users/paulmck/scalability/paper/BareMetal.2015.01.15b.pdf">did come as a surprise [PDF]</a>. -RCU has made good progress towards meeting this requirement, even -for context-switch-have <tt>CONFIG_NO_HZ_FULL=y</tt> workloads, -but there is room for further improvement. - -<h3><a name="Tracing and RCU">Tracing and RCU</a></h3> - -<p> -It is possible to use tracing on RCU code, but tracing itself -uses RCU. -For this reason, <tt>rcu_dereference_raw_notrace()</tt> -is provided for use by tracing, which avoids the destructive -recursion that could otherwise ensue. -This API is also used by virtualization in some architectures, -where RCU readers execute in environments in which tracing -cannot be used. -The tracing folks both located the requirement and provided the -needed fix, so this surprise requirement was relatively painless. - -<h3><a name="Energy Efficiency">Energy Efficiency</a></h3> - -<p> -Interrupting idle CPUs is considered socially unacceptable, -especially by people with battery-powered embedded systems. -RCU therefore conserves energy by detecting which CPUs are -idle, including tracking CPUs that have been interrupted from idle. -This is a large part of the energy-efficiency requirement, -so I learned of this via an irate phone call. - -<p> -Because RCU avoids interrupting idle CPUs, it is illegal to -execute an RCU read-side critical section on an idle CPU. -(Kernels built with <tt>CONFIG_PROVE_RCU=y</tt> will splat -if you try it.) -The <tt>RCU_NONIDLE()</tt> macro and <tt>_rcuidle</tt> -event tracing is provided to work around this restriction. -In addition, <tt>rcu_is_watching()</tt> may be used to -test whether or not it is currently legal to run RCU read-side -critical sections on this CPU. -I learned of the need for diagnostics on the one hand -and <tt>RCU_NONIDLE()</tt> on the other while inspecting -idle-loop code. -Steven Rostedt supplied <tt>_rcuidle</tt> event tracing, -which is used quite heavily in the idle loop. - -<p> -It is similarly socially unacceptable to interrupt an -<tt>nohz_full</tt> CPU running in userspace. -RCU must therefore track <tt>nohz_full</tt> userspace -execution. -And in -<a href="https://lwn.net/Articles/558284/"><tt>CONFIG_NO_HZ_FULL_SYSIDLE=y</tt></a> -kernels, RCU must separately track idle CPUs on the one hand and -CPUs that are either idle or executing in userspace on the other. -In both cases, RCU must be able to sample state at two points in -time, and be able to determine whether or not some other CPU spent -any time idle and/or executing in userspace. - -<p> -These energy-efficiency requirements have proven quite difficult to -understand and to meet, for example, there have been more than five -clean-sheet rewrites of RCU's energy-efficiency code, the last of -which was finally able to demonstrate -<a href="http://www.rdrop.com/users/paulmck/realtime/paper/AMPenergy.2013.04.19a.pdf">real energy savings running on real hardware [PDF]</a>. -As noted earlier, -I learned of many of these requirements via angry phone calls: -Flaming me on the Linux-kernel mailing list was apparently not -sufficient to fully vent their ire at RCU's energy-efficiency bugs! - -<h3><a name="Memory Efficiency">Memory Efficiency</a></h3> - -<p> -Although small-memory non-realtime systems can simply use Tiny RCU, -code size is only one aspect of memory efficiency. -Another aspect is the size of the <tt>rcu_head</tt> structure -used by <tt>call_rcu()</tt> and <tt>kfree_rcu()</tt>. -Although this structure contains nothing more than a pair of pointers, -it does appear in many RCU-protected data structures, including -some that are size critical. -The <tt>page</tt> structure is a case in point, as evidenced by -the many occurrences of the <tt>union</tt> keyword within that structure. - -<p> -This need for memory efficiency is one reason that RCU uses hand-crafted -singly linked lists to track the <tt>rcu_head</tt> structures that -are waiting for a grace period to elapse. -It is also the reason why <tt>rcu_head</tt> structures do not contain -debug information, such as fields tracking the file and line of the -<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt> that posted them. -Although this information might appear in debug-only kernel builds at some -point, in the meantime, the <tt>->func</tt> field will often provide -the needed debug information. - -<p> -However, in some cases, the need for memory efficiency leads to even -more extreme measures. -Returning to the <tt>page</tt> structure, the <tt>rcu_head</tt> field -shares storage with a great many other structures that are used at -various points in the corresponding page's lifetime. -In order to correctly resolve certain -<a href="https://lkml.kernel.org/g/1439976106-137226-1-git-send-email-kirill.shutemov@linux.intel.com">race conditions</a>, -the Linux kernel's memory-management subsystem needs a particular bit -to remain zero during all phases of grace-period processing, -and that bit happens to map to the bottom bit of the -<tt>rcu_head</tt> structure's <tt>->next</tt> field. -RCU makes this guarantee as long as <tt>call_rcu()</tt> -is used to post the callback, as opposed to <tt>kfree_rcu()</tt> -or some future “lazy” -variant of <tt>call_rcu()</tt> that might one day be created for -energy-efficiency purposes. - -<h3><a name="Performance, Scalability, Response Time, and Reliability"> -Performance, Scalability, Response Time, and Reliability</a></h3> - -<p> -Expanding on the -<a href="#Performance and Scalability">earlier discussion</a>, -RCU is used heavily by hot code paths in performance-critical -portions of the Linux kernel's networking, security, virtualization, -and scheduling code paths. -RCU must therefore use efficient implementations, especially in its -read-side primitives. -To that end, it would be good if preemptible RCU's implementation -of <tt>rcu_read_lock()</tt> could be inlined, however, doing -this requires resolving <tt>#include</tt> issues with the -<tt>task_struct</tt> structure. - -<p> -The Linux kernel supports hardware configurations with up to -4096 CPUs, which means that RCU must be extremely scalable. -Algorithms that involve frequent acquisitions of global locks or -frequent atomic operations on global variables simply cannot be -tolerated within the RCU implementation. -RCU therefore makes heavy use of a combining tree based on the -<tt>rcu_node</tt> structure. -RCU is required to tolerate all CPUs continuously invoking any -combination of RCU's runtime primitives with minimal per-operation -overhead. -In fact, in many cases, increasing load must <i>decrease</i> the -per-operation overhead, witness the batching optimizations for -<tt>synchronize_rcu()</tt>, <tt>call_rcu()</tt>, -<tt>synchronize_rcu_expedited()</tt>, and <tt>rcu_barrier()</tt>. -As a general rule, RCU must cheerfully accept whatever the -rest of the Linux kernel decides to throw at it. - -<p> -The Linux kernel is used for real-time workloads, especially -in conjunction with the -<a href="https://rt.wiki.kernel.org/index.php/Main_Page">-rt patchset</a>. -The real-time-latency response requirements are such that the -traditional approach of disabling preemption across RCU -read-side critical sections is inappropriate. -Kernels built with <tt>CONFIG_PREEMPT=y</tt> therefore -use an RCU implementation that allows RCU read-side critical -sections to be preempted. -This requirement made its presence known after users made it -clear that an earlier -<a href="https://lwn.net/Articles/107930/">real-time patch</a> -did not meet their needs, in conjunction with some -<a href="https://lkml.kernel.org/g/20050318002026.GA2693@us.ibm.com">RCU issues</a> -encountered by a very early version of the -rt patchset. - -<p> -In addition, RCU must make do with a sub-100-microsecond real-time latency -budget. -In fact, on smaller systems with the -rt patchset, the Linux kernel -provides sub-20-microsecond real-time latencies for the whole kernel, -including RCU. -RCU's scalability and latency must therefore be sufficient for -these sorts of configurations. -To my surprise, the sub-100-microsecond real-time latency budget -<a href="http://www.rdrop.com/users/paulmck/realtime/paper/bigrt.2013.01.31a.LCA.pdf"> -applies to even the largest systems [PDF]</a>, -up to and including systems with 4096 CPUs. -This real-time requirement motivated the grace-period kthread, which -also simplified handling of a number of race conditions. - -<p> -Finally, RCU's status as a synchronization primitive means that -any RCU failure can result in arbitrary memory corruption that can be -extremely difficult to debug. -This means that RCU must be extremely reliable, which in -practice also means that RCU must have an aggressive stress-test -suite. -This stress-test suite is called <tt>rcutorture</tt>. - -<p> -Although the need for <tt>rcutorture</tt> was no surprise, -the current immense popularity of the Linux kernel is posing -interesting—and perhaps unprecedented—validation -challenges. -To see this, keep in mind that there are well over one billion -instances of the Linux kernel running today, given Android -smartphones, Linux-powered televisions, and servers. -This number can be expected to increase sharply with the advent of -the celebrated Internet of Things. - -<p> -Suppose that RCU contains a race condition that manifests on average -once per million years of runtime. -This bug will be occurring about three times per <i>day</i> across -the installed base. -RCU could simply hide behind hardware error rates, given that no one -should really expect their smartphone to last for a million years. -However, anyone taking too much comfort from this thought should -consider the fact that in most jurisdictions, a successful multi-year -test of a given mechanism, which might include a Linux kernel, -suffices for a number of types of safety-critical certifications. -In fact, rumor has it that the Linux kernel is already being used -in production for safety-critical applications. -I don't know about you, but I would feel quite bad if a bug in RCU -killed someone. -Which might explain my recent focus on validation and verification. - -<h2><a name="Other RCU Flavors">Other RCU Flavors</a></h2> - -<p> -One of the more surprising things about RCU is that there are now -no fewer than five <i>flavors</i>, or API families. -In addition, the primary flavor that has been the sole focus up to -this point has two different implementations, non-preemptible and -preemptible. -The other four flavors are listed below, with requirements for each -described in a separate section. - -<ol> -<li> <a href="#Bottom-Half Flavor">Bottom-Half Flavor</a> -<li> <a href="#Sched Flavor">Sched Flavor</a> -<li> <a href="#Sleepable RCU">Sleepable RCU</a> -<li> <a href="#Tasks RCU">Tasks RCU</a> -</ol> - -<h3><a name="Bottom-Half Flavor">Bottom-Half Flavor</a></h3> - -<p> -The softirq-disable (AKA “bottom-half”, -hence the “_bh” abbreviations) -flavor of RCU, or <i>RCU-bh</i>, was developed by -Dipankar Sarma to provide a flavor of RCU that could withstand the -network-based denial-of-service attacks researched by Robert -Olsson. -These attacks placed so much networking load on the system -that some of the CPUs never exited softirq execution, -which in turn prevented those CPUs from ever executing a context switch, -which, in the RCU implementation of that time, prevented grace periods -from ever ending. -The result was an out-of-memory condition and a system hang. - -<p> -The solution was the creation of RCU-bh, which does -<tt>local_bh_disable()</tt> -across its read-side critical sections, and which uses the transition -from one type of softirq processing to another as a quiescent state -in addition to context switch, idle, user mode, and offline. -This means that RCU-bh grace periods can complete even when some of -the CPUs execute in softirq indefinitely, thus allowing algorithms -based on RCU-bh to withstand network-based denial-of-service attacks. - -<p> -Because -<tt>rcu_read_lock_bh()</tt> and <tt>rcu_read_unlock_bh()</tt> -disable and re-enable softirq handlers, any attempt to start a softirq -handlers during the -RCU-bh read-side critical section will be deferred. -In this case, <tt>rcu_read_unlock_bh()</tt> -will invoke softirq processing, which can take considerable time. -One can of course argue that this softirq overhead should be associated -with the code following the RCU-bh read-side critical section rather -than <tt>rcu_read_unlock_bh()</tt>, but the fact -is that most profiling tools cannot be expected to make this sort -of fine distinction. -For example, suppose that a three-millisecond-long RCU-bh read-side -critical section executes during a time of heavy networking load. -There will very likely be an attempt to invoke at least one softirq -handler during that three milliseconds, but any such invocation will -be delayed until the time of the <tt>rcu_read_unlock_bh()</tt>. -This can of course make it appear at first glance as if -<tt>rcu_read_unlock_bh()</tt> was executing very slowly. - -<p> -The -<a href="https://lwn.net/Articles/609973/#RCU Per-Flavor API Table">RCU-bh API</a> -includes -<tt>rcu_read_lock_bh()</tt>, -<tt>rcu_read_unlock_bh()</tt>, -<tt>rcu_dereference_bh()</tt>, -<tt>rcu_dereference_bh_check()</tt>, -<tt>synchronize_rcu_bh()</tt>, -<tt>synchronize_rcu_bh_expedited()</tt>, -<tt>call_rcu_bh()</tt>, -<tt>rcu_barrier_bh()</tt>, and -<tt>rcu_read_lock_bh_held()</tt>. - -<h3><a name="Sched Flavor">Sched Flavor</a></h3> - -<p> -Before preemptible RCU, waiting for an RCU grace period had the -side effect of also waiting for all pre-existing interrupt -and NMI handlers. -However, there are legitimate preemptible-RCU implementations that -do not have this property, given that any point in the code outside -of an RCU read-side critical section can be a quiescent state. -Therefore, <i>RCU-sched</i> was created, which follows “classic” -RCU in that an RCU-sched grace period waits for for pre-existing -interrupt and NMI handlers. -In kernels built with <tt>CONFIG_PREEMPT=n</tt>, the RCU and RCU-sched -APIs have identical implementations, while kernels built with -<tt>CONFIG_PREEMPT=y</tt> provide a separate implementation for each. - -<p> -Note well that in <tt>CONFIG_PREEMPT=y</tt> kernels, -<tt>rcu_read_lock_sched()</tt> and <tt>rcu_read_unlock_sched()</tt> -disable and re-enable preemption, respectively. -This means that if there was a preemption attempt during the -RCU-sched read-side critical section, <tt>rcu_read_unlock_sched()</tt> -will enter the scheduler, with all the latency and overhead entailed. -Just as with <tt>rcu_read_unlock_bh()</tt>, this can make it look -as if <tt>rcu_read_unlock_sched()</tt> was executing very slowly. -However, the highest-priority task won't be preempted, so that task -will enjoy low-overhead <tt>rcu_read_unlock_sched()</tt> invocations. - -<p> -The -<a href="https://lwn.net/Articles/609973/#RCU Per-Flavor API Table">RCU-sched API</a> -includes -<tt>rcu_read_lock_sched()</tt>, -<tt>rcu_read_unlock_sched()</tt>, -<tt>rcu_read_lock_sched_notrace()</tt>, -<tt>rcu_read_unlock_sched_notrace()</tt>, -<tt>rcu_dereference_sched()</tt>, -<tt>rcu_dereference_sched_check()</tt>, -<tt>synchronize_sched()</tt>, -<tt>synchronize_rcu_sched_expedited()</tt>, -<tt>call_rcu_sched()</tt>, -<tt>rcu_barrier_sched()</tt>, and -<tt>rcu_read_lock_sched_held()</tt>. -However, anything that disables preemption also marks an RCU-sched -read-side critical section, including -<tt>preempt_disable()</tt> and <tt>preempt_enable()</tt>, -<tt>local_irq_save()</tt> and <tt>local_irq_restore()</tt>, -and so on. - -<h3><a name="Sleepable RCU">Sleepable RCU</a></h3> - -<p> -For well over a decade, someone saying “I need to block within -an RCU read-side critical section” was a reliable indication -that this someone did not understand RCU. -After all, if you are always blocking in an RCU read-side critical -section, you can probably afford to use a higher-overhead synchronization -mechanism. -However, that changed with the advent of the Linux kernel's notifiers, -whose RCU read-side critical -sections almost never sleep, but sometimes need to. -This resulted in the introduction of -<a href="https://lwn.net/Articles/202847/">sleepable RCU</a>, -or <i>SRCU</i>. - -<p> -SRCU allows different domains to be defined, with each such domain -defined by an instance of an <tt>srcu_struct</tt> structure. -A pointer to this structure must be passed in to each SRCU function, -for example, <tt>synchronize_srcu(&ss)</tt>, where -<tt>ss</tt> is the <tt>srcu_struct</tt> structure. -The key benefit of these domains is that a slow SRCU reader in one -domain does not delay an SRCU grace period in some other domain. -That said, one consequence of these domains is that read-side code -must pass a “cookie” from <tt>srcu_read_lock()</tt> -to <tt>srcu_read_unlock()</tt>, for example, as follows: - -<blockquote> -<pre> - 1 int idx; - 2 - 3 idx = srcu_read_lock(&ss); - 4 do_something(); - 5 srcu_read_unlock(&ss, idx); -</pre> -</blockquote> - -<p> -As noted above, it is legal to block within SRCU read-side critical sections, -however, with great power comes great responsibility. -If you block forever in one of a given domain's SRCU read-side critical -sections, then that domain's grace periods will also be blocked forever. -Of course, one good way to block forever is to deadlock, which can -happen if any operation in a given domain's SRCU read-side critical -section can block waiting, either directly or indirectly, for that domain's -grace period to elapse. -For example, this results in a self-deadlock: - -<blockquote> -<pre> - 1 int idx; - 2 - 3 idx = srcu_read_lock(&ss); - 4 do_something(); - 5 synchronize_srcu(&ss); - 6 srcu_read_unlock(&ss, idx); -</pre> -</blockquote> - -<p> -However, if line 5 acquired a mutex that was held across -a <tt>synchronize_srcu()</tt> for domain <tt>ss</tt>, -deadlock would still be possible. -Furthermore, if line 5 acquired a mutex that was held across -a <tt>synchronize_srcu()</tt> for some other domain <tt>ss1</tt>, -and if an <tt>ss1</tt>-domain SRCU read-side critical section -acquired another mutex that was held across as <tt>ss</tt>-domain -<tt>synchronize_srcu()</tt>, -deadlock would again be possible. -Such a deadlock cycle could extend across an arbitrarily large number -of different SRCU domains. -Again, with great power comes great responsibility. - -<p> -Unlike the other RCU flavors, SRCU read-side critical sections can -run on idle and even offline CPUs. -This ability requires that <tt>srcu_read_lock()</tt> and -<tt>srcu_read_unlock()</tt> contain memory barriers, which means -that SRCU readers will run a bit slower than would RCU readers. -It also motivates the <tt>smp_mb__after_srcu_read_unlock()</tt> -API, which, in combination with <tt>srcu_read_unlock()</tt>, -guarantees a full memory barrier. - -<p> -The -<a href="https://lwn.net/Articles/609973/#RCU Per-Flavor API Table">SRCU API</a> -includes -<tt>srcu_read_lock()</tt>, -<tt>srcu_read_unlock()</tt>, -<tt>srcu_dereference()</tt>, -<tt>srcu_dereference_check()</tt>, -<tt>synchronize_srcu()</tt>, -<tt>synchronize_srcu_expedited()</tt>, -<tt>call_srcu()</tt>, -<tt>srcu_barrier()</tt>, and -<tt>srcu_read_lock_held()</tt>. -It also includes -<tt>DEFINE_SRCU()</tt>, -<tt>DEFINE_STATIC_SRCU()</tt>, and -<tt>init_srcu_struct()</tt> -APIs for defining and initializing <tt>srcu_struct</tt> structures. - -<h3><a name="Tasks RCU">Tasks RCU</a></h3> - -<p> -Some forms of tracing use “tramopolines” to handle the -binary rewriting required to install different types of probes. -It would be good to be able to free old trampolines, which sounds -like a job for some form of RCU. -However, because it is necessary to be able to install a trace -anywhere in the code, it is not possible to use read-side markers -such as <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>. -In addition, it does not work to have these markers in the trampoline -itself, because there would need to be instructions following -<tt>rcu_read_unlock()</tt>. -Although <tt>synchronize_rcu()</tt> would guarantee that execution -reached the <tt>rcu_read_unlock()</tt>, it would not be able to -guarantee that execution had completely left the trampoline. - -<p> -The solution, in the form of -<a href="https://lwn.net/Articles/607117/"><i>Tasks RCU</i></a>, -is to have implicit -read-side critical sections that are delimited by voluntary context -switches, that is, calls to <tt>schedule()</tt>, -<tt>cond_resched_rcu_qs()</tt>, and -<tt>synchronize_rcu_tasks()</tt>. -In addition, transitions to and from userspace execution also delimit -tasks-RCU read-side critical sections. - -<p> -The tasks-RCU API is quite compact, consisting only of -<tt>call_rcu_tasks()</tt>, -<tt>synchronize_rcu_tasks()</tt>, and -<tt>rcu_barrier_tasks()</tt>. - -<h2><a name="Possible Future Changes">Possible Future Changes</a></h2> - -<p> -One of the tricks that RCU uses to attain update-side scalability is -to increase grace-period latency with increasing numbers of CPUs. -If this becomes a serious problem, it will be necessary to rework the -grace-period state machine so as to avoid the need for the additional -latency. - -<p> -Expedited grace periods scan the CPUs, so their latency and overhead -increases with increasing numbers of CPUs. -If this becomes a serious problem on large systems, it will be necessary -to do some redesign to avoid this scalability problem. - -<p> -RCU disables CPU hotplug in a few places, perhaps most notably in the -expedited grace-period and <tt>rcu_barrier()</tt> operations. -If there is a strong reason to use expedited grace periods in CPU-hotplug -notifiers, it will be necessary to avoid disabling CPU hotplug. -This would introduce some complexity, so there had better be a <i>very</i> -good reason. - -<p> -The tradeoff between grace-period latency on the one hand and interruptions -of other CPUs on the other hand may need to be re-examined. -The desire is of course for zero grace-period latency as well as zero -interprocessor interrupts undertaken during an expedited grace period -operation. -While this ideal is unlikely to be achievable, it is quite possible that -further improvements can be made. - -<p> -The multiprocessor implementations of RCU use a combining tree that -groups CPUs so as to reduce lock contention and increase cache locality. -However, this combining tree does not spread its memory across NUMA -nodes nor does it align the CPU groups with hardware features such -as sockets or cores. -Such spreading and alignment is currently believed to be unnecessary -because the hotpath read-side primitives do not access the combining -tree, nor does <tt>call_rcu()</tt> in the common case. -If you believe that your architecture needs such spreading and alignment, -then your architecture should also benefit from the -<tt>rcutree.rcu_fanout_leaf</tt> boot parameter, which can be set -to the number of CPUs in a socket, NUMA node, or whatever. -If the number of CPUs is too large, use a fraction of the number of -CPUs. -If the number of CPUs is a large prime number, well, that certainly -is an “interesting” architectural choice! -More flexible arrangements might be considered, but only if -<tt>rcutree.rcu_fanout_leaf</tt> has proven inadequate, and only -if the inadequacy has been demonstrated by a carefully run and -realistic system-level workload. - -<p> -Please note that arrangements that require RCU to remap CPU numbers will -require extremely good demonstration of need and full exploration of -alternatives. - -<p> -There is an embarrassingly large number of flavors of RCU, and this -number has been increasing over time. -Perhaps it will be possible to combine some at some future date. - -<p> -RCU's various kthreads are reasonably recent additions. -It is quite likely that adjustments will be required to more gracefully -handle extreme loads. -It might also be necessary to be able to relate CPU utilization by -RCU's kthreads and softirq handlers to the code that instigated this -CPU utilization. -For example, RCU callback overhead might be charged back to the -originating <tt>call_rcu()</tt> instance, though probably not -in production kernels. - -<h2><a name="Summary">Summary</a></h2> - -<p> -This document has presented more than two decade's worth of RCU -requirements. -Given that the requirements keep changing, this will not be the last -word on this subject, but at least it serves to get an important -subset of the requirements set forth. - -<h2><a name="Acknowledgments">Acknowledgments</a></h2> - -I am grateful to Steven Rostedt, Lai Jiangshan, Ingo Molnar, -Oleg Nesterov, Borislav Petkov, Peter Zijlstra, Boqun Feng, and -Andy Lutomirski for their help in rendering -this article human readable, and to Michelle Rankin for her support -of this effort. -Other contributions are acknowledged in the Linux kernel's git archive. -The cartoon is copyright (c) 2013 by Melissa Broussard, -and is provided -under the terms of the Creative Commons Attribution-Share Alike 3.0 -United States license. - -<p>@@QQAL@@ - -</body></html> diff --git a/Documentation/RCU/Design/htmlqqz.sh b/Documentation/RCU/Design/htmlqqz.sh deleted file mode 100755 index d354f069559b..000000000000 --- a/Documentation/RCU/Design/htmlqqz.sh +++ /dev/null @@ -1,108 +0,0 @@ -#!/bin/sh -# -# Usage: sh htmlqqz.sh file -# -# Extracts and converts quick quizzes in a proto-HTML document file.htmlx. -# Commands, all of which must be on a line by themselves: -# -# "<p>@@QQ@@": Start of a quick quiz. -# "<p>@@QQA@@": Start of a quick-quiz answer. -# "<p>@@QQE@@": End of a quick-quiz answer, and thus of the quick quiz. -# "<p>@@QQAL@@": Place to put quick-quiz answer list. -# -# Places the result in file.html. -# -# This program is free software; you can redistribute it and/or modify -# it under the terms of the GNU General Public License as published by -# the Free Software Foundation; either version 2 of the License, or -# (at your option) any later version. -# -# This program is distributed in the hope that it will be useful, -# but WITHOUT ANY WARRANTY; without even the implied warranty of -# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the -# GNU General Public License for more details. -# -# You should have received a copy of the GNU General Public License -# along with this program; if not, you can access it online at -# http://www.gnu.org/licenses/gpl-2.0.html. -# -# Copyright (c) 2013 Paul E. McKenney, IBM Corporation. - -fn=$1 -if test ! -r $fn.htmlx -then - echo "Error: $fn.htmlx unreadable." - exit 1 -fi - -echo "<!-- DO NOT HAND EDIT. -->" > $fn.html -echo "<!-- Instead, edit $fn.htmlx and run 'sh htmlqqz.sh $fn' -->" >> $fn.html -awk < $fn.htmlx >> $fn.html ' - -state == "" && $1 != "<p>@@QQ@@" && $1 != "<p>@@QQAL@@" { - print $0; - if ($0 ~ /^<p>@@QQ/) - print "Bad Quick Quiz command: " NR " (expected <p>@@QQ@@ or <p>@@QQAL@@)." > "/dev/stderr" - next; -} - -state == "" && $1 == "<p>@@QQ@@" { - qqn++; - qqlineno = NR; - haveqq = 1; - state = "qq"; - print "<p><a name=\"Quick Quiz " qqn "\"><b>Quick Quiz " qqn "</b>:</a>" - next; -} - -state == "qq" && $1 != "<p>@@QQA@@" { - qq[qqn] = qq[qqn] $0 "\n"; - print $0 - if ($0 ~ /^<p>@@QQ/) - print "Bad Quick Quiz command: " NR ". (expected <p>@@QQA@@)" > "/dev/stderr" - next; -} - -state == "qq" && $1 == "<p>@@QQA@@" { - state = "qqa"; - print "<br><a href=\"#qq" qqn "answer\">Answer</a>" - next; -} - -state == "qqa" && $1 != "<p>@@QQE@@" { - qqa[qqn] = qqa[qqn] $0 "\n"; - if ($0 ~ /^<p>@@QQ/) - print "Bad Quick Quiz command: " NR " (expected <p>@@QQE@@)." > "/dev/stderr" - next; -} - -state == "qqa" && $1 == "<p>@@QQE@@" { - state = ""; - next; -} - -state == "" && $1 == "<p>@@QQAL@@" { - haveqq = ""; - print "<h3><a name=\"Answers to Quick Quizzes\">" - print "Answers to Quick Quizzes</a></h3>" - print ""; - for (i = 1; i <= qqn; i++) { - print "<a name=\"qq" i "answer\"></a>" - print "<p><b>Quick Quiz " i "</b>:" - print qq[i]; - print ""; - print "</p><p><b>Answer</b>:" - print qqa[i]; - print ""; - print "</p><p><a href=\"#Quick%20Quiz%20" i "\"><b>Back to Quick Quiz " i "</b>.</a>" - print ""; - } - next; -} - -END { - if (state != "") - print "Unterminated Quick Quiz: " qqlineno "." > "/dev/stderr" - else if (haveqq) - print "Missing \"<p>@@QQAL@@\", no Quick Quiz." > "/dev/stderr" -}' diff --git a/Documentation/RCU/trace.txt b/Documentation/RCU/trace.txt index ec6998b1b6d0..00a3a38b375a 100644 --- a/Documentation/RCU/trace.txt +++ b/Documentation/RCU/trace.txt @@ -237,17 +237,17 @@ o "ktl" is the low-order 16 bits (in hexadecimal) of the count of The output of "cat rcu/rcu_preempt/rcuexp" looks as follows: -s=21872 wd0=0 wd1=0 wd2=0 wd3=5 n=0 enq=0 sc=21872 +s=21872 wd1=0 wd2=0 wd3=5 n=0 enq=0 sc=21872 These fields are as follows: o "s" is the sequence number, with an odd number indicating that an expedited grace period is in progress. -o "wd0", "wd1", "wd2", and "wd3" are the number of times that an - attempt to start an expedited grace period found that someone - else had completed an expedited grace period that satisfies the - attempted request. "Our work is done." +o "wd1", "wd2", and "wd3" are the number of times that an attempt + to start an expedited grace period found that someone else had + completed an expedited grace period that satisfies the attempted + request. "Our work is done." o "n" is number of times that a concurrent CPU-hotplug operation forced a fallback to a normal grace period. diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt index dc49c6712b17..111770ffa10e 100644 --- a/Documentation/RCU/whatisRCU.txt +++ b/Documentation/RCU/whatisRCU.txt @@ -681,22 +681,30 @@ Although RCU can be used in many different ways, a very common use of RCU is analogous to reader-writer locking. The following unified diff shows how closely related RCU and reader-writer locking can be. + @@ -5,5 +5,5 @@ struct el { + int data; + /* Other data fields */ + }; + -rwlock_t listmutex; + +spinlock_t listmutex; + struct el head; + @@ -13,15 +14,15 @@ struct list_head *lp; struct el *p; - - read_lock(); + - read_lock(&listmutex); - list_for_each_entry(p, head, lp) { + rcu_read_lock(); + list_for_each_entry_rcu(p, head, lp) { if (p->key == key) { *result = p->data; - - read_unlock(); + - read_unlock(&listmutex); + rcu_read_unlock(); return 1; } } - - read_unlock(); + - read_unlock(&listmutex); + rcu_read_unlock(); return 0; } @@ -732,7 +740,7 @@ Or, for those who prefer a side-by-side listing: 5 int data; 5 int data; 6 /* Other data fields */ 6 /* Other data fields */ 7 }; 7 }; - 8 spinlock_t listmutex; 8 spinlock_t listmutex; + 8 rwlock_t listmutex; 8 spinlock_t listmutex; 9 struct el head; 9 struct el head; 1 int search(long key, int *result) 1 int search(long key, int *result) @@ -740,15 +748,15 @@ Or, for those who prefer a side-by-side listing: 3 struct list_head *lp; 3 struct list_head *lp; 4 struct el *p; 4 struct el *p; 5 5 - 6 read_lock(); 6 rcu_read_lock(); + 6 read_lock(&listmutex); 6 rcu_read_lock(); 7 list_for_each_entry(p, head, lp) { 7 list_for_each_entry_rcu(p, head, lp) { 8 if (p->key == key) { 8 if (p->key == key) { 9 *result = p->data; 9 *result = p->data; -10 read_unlock(); 10 rcu_read_unlock(); +10 read_unlock(&listmutex); 10 rcu_read_unlock(); 11 return 1; 11 return 1; 12 } 12 } 13 } 13 } -14 read_unlock(); 14 rcu_read_unlock(); +14 read_unlock(&listmutex); 14 rcu_read_unlock(); 15 return 0; 15 return 0; 16 } 16 } diff --git a/Documentation/arm64/booting.txt b/Documentation/arm64/booting.txt index 56d6d8b796db..8d0df62c3fe0 100644 --- a/Documentation/arm64/booting.txt +++ b/Documentation/arm64/booting.txt @@ -132,6 +132,10 @@ NOTE: versions prior to v4.6 cannot make use of memory below the physical offset of the Image so it is recommended that the Image be placed as close as possible to the start of system RAM. +If an initrd/initramfs is passed to the kernel at boot, it must reside +entirely within a 1 GB aligned physical memory window of up to 32 GB in +size that fully covers the kernel Image as well. + Any memory described to the kernel (even that below the start of the image) which is not marked as reserved from the kernel (e.g., with a memreserve region in the device tree) will be considered as available to diff --git a/Documentation/devicetree/bindings/arm/altera/socfpga-eccmgr.txt b/Documentation/devicetree/bindings/arm/altera/socfpga-eccmgr.txt index 885f93d14ef9..5a6b16070a33 100644 --- a/Documentation/devicetree/bindings/arm/altera/socfpga-eccmgr.txt +++ b/Documentation/devicetree/bindings/arm/altera/socfpga-eccmgr.txt @@ -3,6 +3,7 @@ This driver uses the EDAC framework to implement the SOCFPGA ECC Manager. The ECC Manager counts and corrects single bit errors and counts/handles double bit errors which are uncorrectable. +Cyclone5 and Arria5 ECC Manager Required Properties: - compatible : Should be "altr,socfpga-ecc-manager" - #address-cells: must be 1 @@ -47,3 +48,52 @@ Example: interrupts = <0 178 1>, <0 179 1>; }; }; + +Arria10 SoCFPGA ECC Manager +The Arria10 SoC ECC Manager handles the IRQs for each peripheral +in a shared register instead of individual IRQs like the Cyclone5 +and Arria5. Therefore the device tree is different as well. + +Required Properties: +- compatible : Should be "altr,socfpga-a10-ecc-manager" +- altr,sysgr-syscon : phandle to Arria10 System Manager Block + containing the ECC manager registers. +- #address-cells: must be 1 +- #size-cells: must be 1 +- interrupts : Should be single bit error interrupt, then double bit error + interrupt. Note the rising edge type. +- ranges : standard definition, should translate from local addresses + +Subcomponents: + +L2 Cache ECC +Required Properties: +- compatible : Should be "altr,socfpga-a10-l2-ecc" +- reg : Address and size for ECC error interrupt clear registers. + +On-Chip RAM ECC +Required Properties: +- compatible : Should be "altr,socfpga-a10-ocram-ecc" +- reg : Address and size for ECC block registers. + +Example: + + eccmgr: eccmgr@ffd06000 { + compatible = "altr,socfpga-a10-ecc-manager"; + altr,sysmgr-syscon = <&sysmgr>; + #address-cells = <1>; + #size-cells = <1>; + interrupts = <0 2 IRQ_TYPE_LEVEL_HIGH>, + <0 0 IRQ_TYPE_LEVEL_HIGH>; + ranges; + + l2-ecc@ffd06010 { + compatible = "altr,socfpga-a10-l2-ecc"; + reg = <0xffd06010 0x4>; + }; + + ocram-ecc@ff8c3000 { + compatible = "altr,socfpga-a10-ocram-ecc"; + reg = <0xff8c3000 0x90>; + }; + }; diff --git a/Documentation/devicetree/bindings/arm/pmu.txt b/Documentation/devicetree/bindings/arm/pmu.txt index 6eb73be9433e..74d5417d0410 100644 --- a/Documentation/devicetree/bindings/arm/pmu.txt +++ b/Documentation/devicetree/bindings/arm/pmu.txt @@ -22,10 +22,11 @@ Required properties: "arm,arm11mpcore-pmu" "arm,arm1176-pmu" "arm,arm1136-pmu" + "brcm,vulcan-pmu" + "cavium,thunder-pmu" "qcom,scorpion-pmu" "qcom,scorpion-mp-pmu" "qcom,krait-pmu" - "cavium,thunder-pmu" - interrupts : 1 combined interrupt or 1 per core. If the interrupt is a per-cpu interrupt (PPI) then 1 interrupt should be specified. diff --git a/Documentation/devicetree/bindings/devfreq/event/exynos-nocp.txt b/Documentation/devicetree/bindings/devfreq/event/exynos-nocp.txt new file mode 100644 index 000000000000..fd459f00aa5a --- /dev/null +++ b/Documentation/devicetree/bindings/devfreq/event/exynos-nocp.txt @@ -0,0 +1,26 @@ + +* Samsung Exynos NoC (Network on Chip) Probe device + +The Samsung Exynos542x SoC has NoC (Network on Chip) Probe for NoC bus. +NoC provides the primitive values to get the performance data. The packets +that the Network on Chip (NoC) probes detects are transported over +the network infrastructure to observer units. You can configure probes to +capture packets with header or data on the data request response network, +or as traffic debug or statistic collectors. Exynos542x bus has multiple +NoC probes to provide bandwidth information about behavior of the SoC +that you can use while analyzing system performance. + +Required properties: +- compatible: Should be "samsung,exynos5420-nocp" +- reg: physical base address of each NoC Probe and length of memory mapped region. + +Optional properties: +- clock-names : the name of clock used by the NoC Probe, "nocp" +- clocks : phandles for clock specified in "clock-names" property + +Example : NoC Probe nodes in Device Tree are listed below. + + nocp_mem0_0: nocp@10CA1000 { + compatible = "samsung,exynos5420-nocp"; + reg = <0x10CA1000 0x200>; + }; diff --git a/Documentation/devicetree/bindings/devfreq/exynos-bus.txt b/Documentation/devicetree/bindings/devfreq/exynos-bus.txt new file mode 100644 index 000000000000..d3ec8e676b6b --- /dev/null +++ b/Documentation/devicetree/bindings/devfreq/exynos-bus.txt @@ -0,0 +1,409 @@ +* Generic Exynos Bus frequency device + +The Samsung Exynos SoC has many buses for data transfer between DRAM +and sub-blocks in SoC. Most Exynos SoCs share the common architecture +for buses. Generally, each bus of Exynos SoC includes a source clock +and a power line, which are able to change the clock frequency +of the bus in runtime. To monitor the usage of each bus in runtime, +the driver uses the PPMU (Platform Performance Monitoring Unit), which +is able to measure the current load of sub-blocks. + +The Exynos SoC includes the various sub-blocks which have the each AXI bus. +The each AXI bus has the owned source clock but, has not the only owned +power line. The power line might be shared among one more sub-blocks. +So, we can divide into two type of device as the role of each sub-block. +There are two type of bus devices as following: +- parent bus device +- passive bus device + +Basically, parent and passive bus device share the same power line. +The parent bus device can only change the voltage of shared power line +and the rest bus devices (passive bus device) depend on the decision of +the parent bus device. If there are three blocks which share the VDD_xxx +power line, Only one block should be parent device and then the rest blocks +should depend on the parent device as passive device. + + VDD_xxx |--- A block (parent) + |--- B block (passive) + |--- C block (passive) + +There are a little different composition among Exynos SoC because each Exynos +SoC has different sub-blocks. Therefore, such difference should be specified +in devicetree file instead of each device driver. In result, this driver +is able to support the bus frequency for all Exynos SoCs. + +Required properties for all bus devices: +- compatible: Should be "samsung,exynos-bus". +- clock-names : the name of clock used by the bus, "bus". +- clocks : phandles for clock specified in "clock-names" property. +- operating-points-v2: the OPP table including frequency/voltage information + to support DVFS (Dynamic Voltage/Frequency Scaling) feature. + +Required properties only for parent bus device: +- vdd-supply: the regulator to provide the buses with the voltage. +- devfreq-events: the devfreq-event device to monitor the current utilization + of buses. + +Required properties only for passive bus device: +- devfreq: the parent bus device. + +Optional properties only for parent bus device: +- exynos,saturation-ratio: the percentage value which is used to calibrate + the performance count against total cycle count. +- exynos,voltage-tolerance: the percentage value for bus voltage tolerance + which is used to calculate the max voltage. + +Detailed correlation between sub-blocks and power line according to Exynos SoC: +- In case of Exynos3250, there are two power line as following: + VDD_MIF |--- DMC + + VDD_INT |--- LEFTBUS (parent device) + |--- PERIL + |--- MFC + |--- G3D + |--- RIGHTBUS + |--- PERIR + |--- FSYS + |--- LCD0 + |--- PERIR + |--- ISP + |--- CAM + +- In case of Exynos4210, there is one power line as following: + VDD_INT |--- DMC (parent device) + |--- LEFTBUS + |--- PERIL + |--- MFC(L) + |--- G3D + |--- TV + |--- LCD0 + |--- RIGHTBUS + |--- PERIR + |--- MFC(R) + |--- CAM + |--- FSYS + |--- GPS + |--- LCD0 + |--- LCD1 + +- In case of Exynos4x12, there are two power line as following: + VDD_MIF |--- DMC + + VDD_INT |--- LEFTBUS (parent device) + |--- PERIL + |--- MFC(L) + |--- G3D + |--- TV + |--- IMAGE + |--- RIGHTBUS + |--- PERIR + |--- MFC(R) + |--- CAM + |--- FSYS + |--- GPS + |--- LCD0 + |--- ISP + +- In case of Exynos5422, there are two power line as following: + VDD_MIF |--- DREX 0 (parent device, DRAM EXpress controller) + |--- DREX 1 + + VDD_INT |--- NoC_Core (parent device) + |--- G2D + |--- G3D + |--- DISP1 + |--- NoC_WCORE + |--- GSCL + |--- MSCL + |--- ISP + |--- MFC + |--- GEN + |--- PERIS + |--- PERIC + |--- FSYS + |--- FSYS2 + +Example1: + Show the AXI buses of Exynos3250 SoC. Exynos3250 divides the buses to + power line (regulator). The MIF (Memory Interface) AXI bus is used to + transfer data between DRAM and CPU and uses the VDD_MIF regulator. + + - MIF (Memory Interface) block + : VDD_MIF |--- DMC (Dynamic Memory Controller) + + - INT (Internal) block + : VDD_INT |--- LEFTBUS (parent device) + |--- PERIL + |--- MFC + |--- G3D + |--- RIGHTBUS + |--- FSYS + |--- LCD0 + |--- PERIR + |--- ISP + |--- CAM + + - MIF bus's frequency/voltage table + ----------------------- + |Lv| Freq | Voltage | + ----------------------- + |L1| 50000 |800000 | + |L2| 100000 |800000 | + |L3| 134000 |800000 | + |L4| 200000 |825000 | + |L5| 400000 |875000 | + ----------------------- + + - INT bus's frequency/voltage table + ---------------------------------------------------------- + |Block|LEFTBUS|RIGHTBUS|MCUISP |ISP |PERIL ||VDD_INT | + | name| |LCD0 | | | || | + | | |FSYS | | | || | + | | |MFC | | | || | + ---------------------------------------------------------- + |Mode |*parent|passive |passive|passive|passive|| | + ---------------------------------------------------------- + |Lv |Frequency ||Voltage | + ---------------------------------------------------------- + |L1 |50000 |50000 |50000 |50000 |50000 ||900000 | + |L2 |80000 |80000 |80000 |80000 |80000 ||900000 | + |L3 |100000 |100000 |100000 |100000 |100000 ||1000000 | + |L4 |134000 |134000 |200000 |200000 | ||1000000 | + |L5 |200000 |200000 |400000 |300000 | ||1000000 | + ---------------------------------------------------------- + +Example2 : + The bus of DMC (Dynamic Memory Controller) block in exynos3250.dtsi + is listed below: + + bus_dmc: bus_dmc { + compatible = "samsung,exynos-bus"; + clocks = <&cmu_dmc CLK_DIV_DMC>; + clock-names = "bus"; + operating-points-v2 = <&bus_dmc_opp_table>; + status = "disabled"; + }; + + bus_dmc_opp_table: opp_table1 { + compatible = "operating-points-v2"; + opp-shared; + + opp@50000000 { + opp-hz = /bits/ 64 <50000000>; + opp-microvolt = <800000>; + }; + opp@100000000 { + opp-hz = /bits/ 64 <100000000>; + opp-microvolt = <800000>; + }; + opp@134000000 { + opp-hz = /bits/ 64 <134000000>; + opp-microvolt = <800000>; + }; + opp@200000000 { + opp-hz = /bits/ 64 <200000000>; + opp-microvolt = <825000>; + }; + opp@400000000 { + opp-hz = /bits/ 64 <400000000>; + opp-microvolt = <875000>; + }; + }; + + bus_leftbus: bus_leftbus { + compatible = "samsung,exynos-bus"; + clocks = <&cmu CLK_DIV_GDL>; + clock-names = "bus"; + operating-points-v2 = <&bus_leftbus_opp_table>; + status = "disabled"; + }; + + bus_rightbus: bus_rightbus { + compatible = "samsung,exynos-bus"; + clocks = <&cmu CLK_DIV_GDR>; + clock-names = "bus"; + operating-points-v2 = <&bus_leftbus_opp_table>; + status = "disabled"; + }; + + bus_lcd0: bus_lcd0 { + compatible = "samsung,exynos-bus"; + clocks = <&cmu CLK_DIV_ACLK_160>; + clock-names = "bus"; + operating-points-v2 = <&bus_leftbus_opp_table>; + status = "disabled"; + }; + + bus_fsys: bus_fsys { + compatible = "samsung,exynos-bus"; + clocks = <&cmu CLK_DIV_ACLK_200>; + clock-names = "bus"; + operating-points-v2 = <&bus_leftbus_opp_table>; + status = "disabled"; + }; + + bus_mcuisp: bus_mcuisp { + compatible = "samsung,exynos-bus"; + clocks = <&cmu CLK_DIV_ACLK_400_MCUISP>; + clock-names = "bus"; + operating-points-v2 = <&bus_mcuisp_opp_table>; + status = "disabled"; + }; + + bus_isp: bus_isp { + compatible = "samsung,exynos-bus"; + clocks = <&cmu CLK_DIV_ACLK_266>; + clock-names = "bus"; + operating-points-v2 = <&bus_isp_opp_table>; + status = "disabled"; + }; + + bus_peril: bus_peril { + compatible = "samsung,exynos-bus"; + clocks = <&cmu CLK_DIV_ACLK_100>; + clock-names = "bus"; + operating-points-v2 = <&bus_peril_opp_table>; + status = "disabled"; + }; + + bus_mfc: bus_mfc { + compatible = "samsung,exynos-bus"; + clocks = <&cmu CLK_SCLK_MFC>; + clock-names = "bus"; + operating-points-v2 = <&bus_leftbus_opp_table>; + status = "disabled"; + }; + + bus_leftbus_opp_table: opp_table1 { + compatible = "operating-points-v2"; + opp-shared; + + opp@50000000 { + opp-hz = /bits/ 64 <50000000>; + opp-microvolt = <900000>; + }; + opp@80000000 { + opp-hz = /bits/ 64 <80000000>; + opp-microvolt = <900000>; + }; + opp@100000000 { + opp-hz = /bits/ 64 <100000000>; + opp-microvolt = <1000000>; + }; + opp@134000000 { + opp-hz = /bits/ 64 <134000000>; + opp-microvolt = <1000000>; + }; + opp@200000000 { + opp-hz = /bits/ 64 <200000000>; + opp-microvolt = <1000000>; + }; + }; + + bus_mcuisp_opp_table: opp_table2 { + compatible = "operating-points-v2"; + opp-shared; + + opp@50000000 { + opp-hz = /bits/ 64 <50000000>; + }; + opp@80000000 { + opp-hz = /bits/ 64 <80000000>; + }; + opp@100000000 { + opp-hz = /bits/ 64 <100000000>; + }; + opp@200000000 { + opp-hz = /bits/ 64 <200000000>; + }; + opp@400000000 { + opp-hz = /bits/ 64 <400000000>; + }; + }; + + bus_isp_opp_table: opp_table3 { + compatible = "operating-points-v2"; + opp-shared; + + opp@50000000 { + opp-hz = /bits/ 64 <50000000>; + }; + opp@80000000 { + opp-hz = /bits/ 64 <80000000>; + }; + opp@100000000 { + opp-hz = /bits/ 64 <100000000>; + }; + opp@200000000 { + opp-hz = /bits/ 64 <200000000>; + }; + opp@300000000 { + opp-hz = /bits/ 64 <300000000>; + }; + }; + + bus_peril_opp_table: opp_table4 { + compatible = "operating-points-v2"; + opp-shared; + + opp@50000000 { + opp-hz = /bits/ 64 <50000000>; + }; + opp@80000000 { + opp-hz = /bits/ 64 <80000000>; + }; + opp@100000000 { + opp-hz = /bits/ 64 <100000000>; + }; + }; + + + Usage case to handle the frequency and voltage of bus on runtime + in exynos3250-rinato.dts is listed below: + + &bus_dmc { + devfreq-events = <&ppmu_dmc0_3>, <&ppmu_dmc1_3>; + vdd-supply = <&buck1_reg>; /* VDD_MIF */ + status = "okay"; + }; + + &bus_leftbus { + devfreq-events = <&ppmu_leftbus_3>, <&ppmu_rightbus_3>; + vdd-supply = <&buck3_reg>; + status = "okay"; + }; + + &bus_rightbus { + devfreq = <&bus_leftbus>; + status = "okay"; + }; + + &bus_lcd0 { + devfreq = <&bus_leftbus>; + status = "okay"; + }; + + &bus_fsys { + devfreq = <&bus_leftbus>; + status = "okay"; + }; + + &bus_mcuisp { + devfreq = <&bus_leftbus>; + status = "okay"; + }; + + &bus_isp { + devfreq = <&bus_leftbus>; + status = "okay"; + }; + + &bus_peril { + devfreq = <&bus_leftbus>; + status = "okay"; + }; + + &bus_mfc { + devfreq = <&bus_leftbus>; + status = "okay"; + }; diff --git a/Documentation/devicetree/bindings/hwmon/ltc2978.txt b/Documentation/devicetree/bindings/hwmon/ltc2978.txt index a7afbf60bb9c..bf2a47bbdc58 100644 --- a/Documentation/devicetree/bindings/hwmon/ltc2978.txt +++ b/Documentation/devicetree/bindings/hwmon/ltc2978.txt @@ -13,6 +13,7 @@ Required properties: * "lltc,ltc3886" * "lltc,ltc3887" * "lltc,ltm2987" + * "lltc,ltm4675" * "lltc,ltm4676" - reg: I2C slave address diff --git a/Documentation/devicetree/bindings/leds/common.txt b/Documentation/devicetree/bindings/leds/common.txt index 68419843e32f..af10678ea2f6 100644 --- a/Documentation/devicetree/bindings/leds/common.txt +++ b/Documentation/devicetree/bindings/leds/common.txt @@ -37,6 +37,9 @@ Optional properties for child nodes: property is mandatory for the LEDs in the non-flash modes (e.g. torch or indicator). +- panic-indicator : This property specifies that the LED should be used, + if at all possible, as a panic indicator. + Required properties for flash LED child nodes: - flash-max-microamp : Maximum flash LED supply current in microamperes. - flash-max-timeout-us : Maximum timeout in microseconds after which the flash diff --git a/Documentation/devicetree/bindings/leds/leds-gpio.txt b/Documentation/devicetree/bindings/leds/leds-gpio.txt index fea1ebfe24a9..cbbeb1850910 100644 --- a/Documentation/devicetree/bindings/leds/leds-gpio.txt +++ b/Documentation/devicetree/bindings/leds/leds-gpio.txt @@ -23,6 +23,8 @@ LED sub-node properties: property is not present. - retain-state-suspended: (optional) The suspend state can be retained.Such as charge-led gpio. +- panic-indicator : (optional) + see Documentation/devicetree/bindings/leds/common.txt Examples: diff --git a/Documentation/devicetree/bindings/mmc/sdhci-st.txt b/Documentation/devicetree/bindings/mmc/sdhci-st.txt index 18d950df2749..88faa91125bf 100644 --- a/Documentation/devicetree/bindings/mmc/sdhci-st.txt +++ b/Documentation/devicetree/bindings/mmc/sdhci-st.txt @@ -38,7 +38,7 @@ Optional properties: - bus-width: Number of data lines. See: Documentation/devicetree/bindings/mmc/mmc.txt. -- max-frequency: Can be 200MHz, 100Mz or 50MHz (default) and used for +- max-frequency: Can be 200MHz, 100Mz or 50MHz (default) and used for configuring the CCONFIG3 in the mmcss. See: Documentation/devicetree/bindings/mmc/mmc.txt. @@ -48,7 +48,7 @@ Optional properties: - vqmmc-supply: Phandle to the regulator dt node, mentioned as the vcc/vdd supply in eMMC/SD specs. -- sd-uhs--sdr50: To enable the SDR50 in the mmcss. +- sd-uhs-sdr50: To enable the SDR50 in the mmcss. See: Documentation/devicetree/bindings/mmc/mmc.txt. - sd-uhs-sdr104: To enable the SDR104 in the mmcss. diff --git a/Documentation/devicetree/bindings/mmc/tmio_mmc.txt b/Documentation/devicetree/bindings/mmc/tmio_mmc.txt index 7fb746dd1a68..0f610d4b5b00 100644 --- a/Documentation/devicetree/bindings/mmc/tmio_mmc.txt +++ b/Documentation/devicetree/bindings/mmc/tmio_mmc.txt @@ -26,3 +26,6 @@ Required properties: Optional properties: - toshiba,mmc-wrprotect-disable: write-protect detection is unavailable +- pinctrl-names: should be "default", "state_uhs" +- pinctrl-0: should contain default/high speed pin ctrl +- pinctrl-1: should contain uhs mode pin ctrl diff --git a/Documentation/devicetree/bindings/mmc/usdhi6rol0.txt b/Documentation/devicetree/bindings/mmc/usdhi6rol0.txt index 8babdaa8623b..6d1b7971d078 100644 --- a/Documentation/devicetree/bindings/mmc/usdhi6rol0.txt +++ b/Documentation/devicetree/bindings/mmc/usdhi6rol0.txt @@ -12,6 +12,12 @@ Optional properties: - vmmc-supply: a phandle of a regulator, supplying Vcc to the card - vqmmc-supply: a phandle of a regulator, supplying VccQ to the card +- pinctrl-names: Can contain a "default" entry and a "state_uhs" + entry. The state_uhs entry is used together with the default + entry when the board requires distinct settings for UHS speeds. + +- pinctrl-N: One property for each name listed in pinctrl-names, see + ../pinctrl/pinctrl-bindings.txt. Additionally any standard mmc bindings from mmc.txt can be used. diff --git a/Documentation/devicetree/bindings/numa.txt b/Documentation/devicetree/bindings/numa.txt new file mode 100644 index 000000000000..21b35053ca5a --- /dev/null +++ b/Documentation/devicetree/bindings/numa.txt @@ -0,0 +1,275 @@ +============================================================================== +NUMA binding description. +============================================================================== + +============================================================================== +1 - Introduction +============================================================================== + +Systems employing a Non Uniform Memory Access (NUMA) architecture contain +collections of hardware resources including processors, memory, and I/O buses, +that comprise what is commonly known as a NUMA node. +Processor accesses to memory within the local NUMA node is generally faster +than processor accesses to memory outside of the local NUMA node. +DT defines interfaces that allow the platform to convey NUMA node +topology information to OS. + +============================================================================== +2 - numa-node-id +============================================================================== + +For the purpose of identification, each NUMA node is associated with a unique +token known as a node id. For the purpose of this binding +a node id is a 32-bit integer. + +A device node is associated with a NUMA node by the presence of a +numa-node-id property which contains the node id of the device. + +Example: + /* numa node 0 */ + numa-node-id = <0>; + + /* numa node 1 */ + numa-node-id = <1>; + +============================================================================== +3 - distance-map +============================================================================== + +The optional device tree node distance-map describes the relative +distance (memory latency) between all numa nodes. + +- compatible : Should at least contain "numa-distance-map-v1". + +- distance-matrix + This property defines a matrix to describe the relative distances + between all numa nodes. + It is represented as a list of node pairs and their relative distance. + + Note: + 1. Each entry represents distance from first node to second node. + The distances are equal in either direction. + 2. The distance from a node to self (local distance) is represented + with value 10 and all internode distance should be represented with + a value greater than 10. + 3. distance-matrix should have entries in lexicographical ascending + order of nodes. + 4. There must be only one device node distance-map which must + reside in the root node. + 5. If the distance-map node is not present, a default + distance-matrix is used. + +Example: + 4 nodes connected in mesh/ring topology as below, + + 0_______20______1 + | | + | | + 20 20 + | | + | | + |_______________| + 3 20 2 + + if relative distance for each hop is 20, + then internode distance would be, + 0 -> 1 = 20 + 1 -> 2 = 20 + 2 -> 3 = 20 + 3 -> 0 = 20 + 0 -> 2 = 40 + 1 -> 3 = 40 + + and dt presentation for this distance matrix is, + + distance-map { + compatible = "numa-distance-map-v1"; + distance-matrix = <0 0 10>, + <0 1 20>, + <0 2 40>, + <0 3 20>, + <1 0 20>, + <1 1 10>, + <1 2 20>, + <1 3 40>, + <2 0 40>, + <2 1 20>, + <2 2 10>, + <2 3 20>, + <3 0 20>, + <3 1 40>, + <3 2 20>, + <3 3 10>; + }; + +============================================================================== +4 - Example dts +============================================================================== + +Dual socket system consists of 2 boards connected through ccn bus and +each board having one socket/soc of 8 cpus, memory and pci bus. + + memory@c00000 { + device_type = "memory"; + reg = <0x0 0xc00000 0x0 0x80000000>; + /* node 0 */ + numa-node-id = <0>; + }; + + memory@10000000000 { + device_type = "memory"; + reg = <0x100 0x0 0x0 0x80000000>; + /* node 1 */ + numa-node-id = <1>; + }; + + cpus { + #address-cells = <2>; + #size-cells = <0>; + + cpu@0 { + device_type = "cpu"; + compatible = "arm,armv8"; + reg = <0x0 0x0>; + enable-method = "psci"; + /* node 0 */ + numa-node-id = <0>; + }; + cpu@1 { + device_type = "cpu"; + compatible = "arm,armv8"; + reg = <0x0 0x1>; + enable-method = "psci"; + numa-node-id = <0>; + }; + cpu@2 { + device_type = "cpu"; + compatible = "arm,armv8"; + reg = <0x0 0x2>; + enable-method = "psci"; + numa-node-id = <0>; + }; + cpu@3 { + device_type = "cpu"; + compatible = "arm,armv8"; + reg = <0x0 0x3>; + enable-method = "psci"; + numa-node-id = <0>; + }; + cpu@4 { + device_type = "cpu"; + compatible = "arm,armv8"; + reg = <0x0 0x4>; + enable-method = "psci"; + numa-node-id = <0>; + }; + cpu@5 { + device_type = "cpu"; + compatible = "arm,armv8"; + reg = <0x0 0x5>; + enable-method = "psci"; + numa-node-id = <0>; + }; + cpu@6 { + device_type = "cpu"; + compatible = "arm,armv8"; + reg = <0x0 0x6>; + enable-method = "psci"; + numa-node-id = <0>; + }; + cpu@7 { + device_type = "cpu"; + compatible = "arm,armv8"; + reg = <0x0 0x7>; + enable-method = "psci"; + numa-node-id = <0>; + }; + cpu@8 { + device_type = "cpu"; + compatible = "arm,armv8"; + reg = <0x0 0x8>; + enable-method = "psci"; + /* node 1 */ + numa-node-id = <1>; + }; + cpu@9 { + device_type = "cpu"; + compatible = "arm,armv8"; + reg = <0x0 0x9>; + enable-method = "psci"; + numa-node-id = <1>; + }; + cpu@a { + device_type = "cpu"; + compatible = "arm,armv8"; + reg = <0x0 0xa>; + enable-method = "psci"; + numa-node-id = <1>; + }; + cpu@b { + device_type = "cpu"; + compatible = "arm,armv8"; + reg = <0x0 0xb>; + enable-method = "psci"; + numa-node-id = <1>; + }; + cpu@c { + device_type = "cpu"; + compatible = "arm,armv8"; + reg = <0x0 0xc>; + enable-method = "psci"; + numa-node-id = <1>; + }; + cpu@d { + device_type = "cpu"; + compatible = "arm,armv8"; + reg = <0x0 0xd>; + enable-method = "psci"; + numa-node-id = <1>; + }; + cpu@e { + device_type = "cpu"; + compatible = "arm,armv8"; + reg = <0x0 0xe>; + enable-method = "psci"; + numa-node-id = <1>; + }; + cpu@f { + device_type = "cpu"; + compatible = "arm,armv8"; + reg = <0x0 0xf>; + enable-method = "psci"; + numa-node-id = <1>; + }; + }; + + pcie0: pcie0@848000000000 { + compatible = "arm,armv8"; + device_type = "pci"; + bus-range = <0 255>; + #size-cells = <2>; + #address-cells = <3>; + reg = <0x8480 0x00000000 0 0x10000000>; /* Configuration space */ + ranges = <0x03000000 0x8010 0x00000000 0x8010 0x00000000 0x70 0x00000000>; + /* node 0 */ + numa-node-id = <0>; + }; + + pcie1: pcie1@948000000000 { + compatible = "arm,armv8"; + device_type = "pci"; + bus-range = <0 255>; + #size-cells = <2>; + #address-cells = <3>; + reg = <0x9480 0x00000000 0 0x10000000>; /* Configuration space */ + ranges = <0x03000000 0x9010 0x00000000 0x9010 0x00000000 0x70 0x00000000>; + /* node 1 */ + numa-node-id = <1>; + }; + + distance-map { + compatible = "numa-distance-map-v1"; + distance-matrix = <0 0 10>, + <0 1 20>, + <1 1 10>; + }; diff --git a/Documentation/devicetree/bindings/power/rockchip-io-domain.txt b/Documentation/devicetree/bindings/power/rockchip-io-domain.txt index c84fb47265eb..d23dc002a87e 100644 --- a/Documentation/devicetree/bindings/power/rockchip-io-domain.txt +++ b/Documentation/devicetree/bindings/power/rockchip-io-domain.txt @@ -37,8 +37,10 @@ Required properties: - "rockchip,rk3368-pmu-io-voltage-domain" for rk3368 pmu-domains - "rockchip,rk3399-io-voltage-domain" for rk3399 - "rockchip,rk3399-pmu-io-voltage-domain" for rk3399 pmu-domains -- rockchip,grf: phandle to the syscon managing the "general register files" +Deprecated properties: +- rockchip,grf: phandle to the syscon managing the "general register files" + Systems should move the io-domains to a sub-node of the grf simple-mfd. You specify supplies using the standard regulator bindings by including a phandle the relevant regulator. All specified supplies must be able diff --git a/Documentation/devicetree/bindings/regmap/regmap.txt b/Documentation/devicetree/bindings/regmap/regmap.txt index e98a9652ccc8..0127be360fe8 100644 --- a/Documentation/devicetree/bindings/regmap/regmap.txt +++ b/Documentation/devicetree/bindings/regmap/regmap.txt @@ -1,50 +1,29 @@ -Device-Tree binding for regmap - -The endianness mode of CPU & Device scenarios: -Index Device Endianness properties ---------------------------------------------------- -1 BE 'big-endian' -2 LE 'little-endian' -3 Native 'native-endian' - -For one device driver, which will run in different scenarios above -on different SoCs using the devicetree, we need one way to simplify -this. +Devicetree binding for regmap Optional properties: -- {big,little,native}-endian: these are boolean properties, if absent - then the implementation will choose a default based on the device - being controlled. These properties are for register values and all - the buffers only. Native endian means that the CPU and device have - the same endianness. -Examples: -Scenario 1 : CPU in LE mode & device in LE mode. -dev: dev@40031000 { - compatible = "name"; - reg = <0x40031000 0x1000>; - ... -}; + little-endian, + big-endian, + native-endian: See common-properties.txt for a definition -Scenario 2 : CPU in LE mode & device in BE mode. -dev: dev@40031000 { - compatible = "name"; - reg = <0x40031000 0x1000>; - ... - big-endian; -}; +Note: +Regmap defaults to little-endian register access on MMIO based +devices, this is by far the most common setting. On CPU +architectures that typically run big-endian operating systems +(e.g. PowerPC), registers can be defined as big-endian and must +be marked that way in the devicetree. -Scenario 3 : CPU in BE mode & device in BE mode. -dev: dev@40031000 { - compatible = "name"; - reg = <0x40031000 0x1000>; - ... -}; +On SoCs that can be operated in both big-endian and little-endian +modes, with a single hardware switch controlling both the endianess +of the CPU and a byteswap for MMIO registers (e.g. many Broadcom MIPS +chips), "native-endian" is used to allow using the same device tree +blob in both cases. -Scenario 4 : CPU in BE mode & device in LE mode. +Examples: +Scenario 1 : a register set in big-endian mode. dev: dev@40031000 { - compatible = "name"; + compatible = "syscon"; reg = <0x40031000 0x1000>; + big-endian; ... - little-endian; }; diff --git a/Documentation/devicetree/bindings/regulator/max8973-regulator.txt b/Documentation/devicetree/bindings/regulator/max8973-regulator.txt index f80ea2fe27e6..c2c68fcc1b41 100644 --- a/Documentation/devicetree/bindings/regulator/max8973-regulator.txt +++ b/Documentation/devicetree/bindings/regulator/max8973-regulator.txt @@ -32,6 +32,13 @@ Optional properties: Enhanced transient response (ETR) will affect the configuration of CKADV. +-junction-warn-millicelsius: u32, junction warning temperature threshold + in millicelsius. If die temperature crosses this level then + device generates the warning interrupts. + +Please note that thermal functionality is only supported on MAX77621. The +supported threshold warning temperature for MAX77621 are 120 degC and 140 degC. + Example: max8973@1b { diff --git a/Documentation/devicetree/bindings/regulator/pv88080.txt b/Documentation/devicetree/bindings/regulator/pv88080.txt new file mode 100644 index 000000000000..38a614210dcb --- /dev/null +++ b/Documentation/devicetree/bindings/regulator/pv88080.txt @@ -0,0 +1,49 @@ +* Powerventure Semiconductor PV88080 Voltage Regulator + +Required properties: +- compatible: "pvs,pv88080". +- reg: I2C slave address, usually 0x49. +- interrupts: the interrupt outputs of the controller +- regulators: A node that houses a sub-node for each regulator within the + device. Each sub-node is identified using the node's name, with valid + values listed below. The content of each sub-node is defined by the + standard binding for regulators; see regulator.txt. + BUCK1, BUCK2, and BUCK3. + +Optional properties: +- Any optional property defined in regulator.txt + +Example + + pmic: pv88080@49 { + compatible = "pvs,pv88080"; + reg = <0x49>; + interrupt-parent = <&gpio>; + interrupts = <24 24>; + + regulators { + BUCK1 { + regulator-name = "buck1"; + regulator-min-microvolt = < 600000>; + regulator-max-microvolt = <1393750>; + regulator-min-microamp = < 220000>; + regulator-max-microamp = <7040000>; + }; + + BUCK2 { + regulator-name = "buck2"; + regulator-min-microvolt = < 600000>; + regulator-max-microvolt = <1393750>; + regulator-min-microamp = <1496000>; + regulator-max-microamp = <4189000>; + }; + + BUCK3 { + regulator-name = "buck3"; + regulator-min-microvolt = <1400000>; + regulator-max-microvolt = <2193750>; + regulator-min-microamp = <1496000>; + regulator-max-microamp = <4189000>; + }; + }; + }; diff --git a/Documentation/devicetree/bindings/regulator/qcom,spmi-regulator.txt b/Documentation/devicetree/bindings/regulator/qcom,spmi-regulator.txt index d00bfd8624a5..46c6f3ed1a1c 100644 --- a/Documentation/devicetree/bindings/regulator/qcom,spmi-regulator.txt +++ b/Documentation/devicetree/bindings/regulator/qcom,spmi-regulator.txt @@ -7,6 +7,7 @@ Qualcomm SPMI Regulators "qcom,pm8841-regulators" "qcom,pm8916-regulators" "qcom,pm8941-regulators" + "qcom,pm8994-regulators" - interrupts: Usage: optional @@ -68,6 +69,37 @@ Qualcomm SPMI Regulators Definition: Reference to regulator supplying the input pin, as described in the data sheet. +- vdd_s1-supply: +- vdd_s2-supply: +- vdd_s3-supply: +- vdd_s4-supply: +- vdd_s5-supply: +- vdd_s6-supply: +- vdd_s7-supply: +- vdd_s8-supply: +- vdd_s9-supply: +- vdd_s10-supply: +- vdd_s11-supply: +- vdd_s12-supply: +- vdd_l1-supply: +- vdd_l2_l26_l28-supply: +- vdd_l3_l11-supply: +- vdd_l4_l27_l31-supply: +- vdd_l5_l7-supply: +- vdd_l6_l12_l32-supply: +- vdd_l8_l16_l30-supply: +- vdd_l9_l10_l18_l22-supply: +- vdd_l13_l19_l23_l24-supply: +- vdd_l14_l15-supply: +- vdd_l17_l29-supply: +- vdd_l20_l21-supply: +- vdd_l25-supply: +- vdd_lvs_1_2-supply: + Usage: optional (pm8994 only) + Value type: <phandle> + Definition: Reference to regulator supplying the input pin, as + described in the data sheet. + The regulator node houses sub-nodes for each regulator within the device. Each sub-node is identified using the node's name, with valid values listed for each @@ -85,6 +117,11 @@ pm8941: l15, l16, l17, l18, l19, l20, l21, l22, l23, l24, lvs1, lvs2, lvs3, mvs1, mvs2 +pm8994: + s1, s2, s3, s4, s5, s6, s7, s8, s9, s10, s11, s12, l1, l2, l3, l4, l5, + l6, l7, l8, l9, l10, l11, l12, l13, l14, l15, l16, l17, l18, l19, l20, + l21, l22, l23, l24, l25, l26, l27, l28, l29, l30, l31, l32, lvs1, lvs2 + The content of each sub-node is defined by the standard binding for regulators - see regulator.txt - with additional custom properties described below: diff --git a/Documentation/devicetree/bindings/regulator/regulator-max77620.txt b/Documentation/devicetree/bindings/regulator/regulator-max77620.txt index b3c8ca672024..1c4bfe786736 100644 --- a/Documentation/devicetree/bindings/regulator/regulator-max77620.txt +++ b/Documentation/devicetree/bindings/regulator/regulator-max77620.txt @@ -94,6 +94,28 @@ Following are additional properties: This is applicable if suspend state FPS source is selected as FPS0, FPS1 or FPS2. +- maxim,ramp-rate-setting: integer, ramp rate(uV/us) setting to be + configured to the device. + The platform may have different ramp + rate than advertised ramp rate if it has + design variation from Maxim's + recommended. On this case, platform + specific ramp rate is used for ramp time + calculation and this property is used + for device register configurations. + The measured ramp rate of platform is + provided by the regulator-ramp-delay + as described in <devicetree/bindings/ + regulator/regulator.txt>. + Maxim Max77620 supports following ramp + delay: + SD: 13.75mV/us, 27.5mV/us, 55mV/us + LDOs: 5mV/us, 100mV/us + +Note: If the measured ramp delay is same as advertised ramp delay then it is not +required to provide the ramp delay with property "maxim,ramp-rate-setting". The +ramp rate can be provided by the regulator-ramp-delay which will be used for +ramp time calculation for voltage change as well as for device configuration. Example: -------- diff --git a/Documentation/devicetree/bindings/regulator/ti-abb-regulator.txt b/Documentation/devicetree/bindings/regulator/ti-abb-regulator.txt index c58db75f959e..c3f6546ebac7 100644 --- a/Documentation/devicetree/bindings/regulator/ti-abb-regulator.txt +++ b/Documentation/devicetree/bindings/regulator/ti-abb-regulator.txt @@ -14,8 +14,8 @@ Required Properties: - "setup-address" - contains setup register address of ABB module (ti,abb-v3) - "int-address" - contains address of interrupt register for ABB module (also see Optional properties) -- #address-cell: should be 0 -- #size-cell: should be 0 +- #address-cells: should be 0 +- #size-cells: should be 0 - clocks: should point to the clock node used by ABB module - ti,settling-time: Settling time in uSecs from SoC documentation for ABB module to settle down(target time for SR2_WTCNT_VALUE). @@ -69,7 +69,7 @@ Example #1: Simplest configuration (no efuse data, hard coded ABB table): abb_x: regulator-abb-x { compatible = "ti,abb-v1"; regulator-name = "abb_x"; - #address-cell = <0>; + #address-cells = <0>; #size-cells = <0>; reg = <0x483072f0 0x8>, <0x48306818 0x4>; reg-names = "base-address", "int-address"; @@ -89,7 +89,7 @@ Example #2: Efuse bits contain ABB mode setting (no LDO override capability) abb_y: regulator-abb-y { compatible = "ti,abb-v2"; regulator-name = "abb_y"; - #address-cell = <0>; + #address-cells = <0>; #size-cells = <0>; reg = <0x4a307bd0 0x8>, <0x4a306014 0x4>, <0x4A002268 0x8>; reg-names = "base-address", "int-address", "efuse-address"; @@ -110,7 +110,7 @@ Example #3: Efuse bits contain ABB mode setting and LDO override capability abb_z: regulator-abb-z { compatible = "ti,abb-v2"; regulator-name = "abb_z"; - #address-cell = <0>; + #address-cells = <0>; #size-cells = <0>; reg = <0x4ae07ce4 0x8>, <0x4ae06010 0x4>, <0x4a002194 0x8>, <0x4ae0C314 0x4>; diff --git a/Documentation/devicetree/bindings/regulator/twl-regulator.txt b/Documentation/devicetree/bindings/regulator/twl-regulator.txt index 75b0c1669504..74a91c4f8530 100644 --- a/Documentation/devicetree/bindings/regulator/twl-regulator.txt +++ b/Documentation/devicetree/bindings/regulator/twl-regulator.txt @@ -57,6 +57,12 @@ For twl4030 regulators/LDOs Optional properties: - Any optional property defined in bindings/regulator/regulator.txt +For twl4030 regulators/LDOs: + - regulator-initial-mode: + - 0x08 - Sleep mode, the nominal output voltage is maintained with low power + consumption with low load current capability. + - 0x0e - Active mode, the regulator can deliver its nominal output voltage + with full-load current capability. Example: diff --git a/Documentation/hwmon/fam15h_power b/Documentation/hwmon/fam15h_power index e2b1b69eebea..fb594c281c46 100644 --- a/Documentation/hwmon/fam15h_power +++ b/Documentation/hwmon/fam15h_power @@ -10,14 +10,22 @@ Supported chips: Datasheets: BIOS and Kernel Developer's Guide (BKDG) For AMD Family 15h Processors BIOS and Kernel Developer's Guide (BKDG) For AMD Family 16h Processors + AMD64 Architecture Programmer's Manual Volume 2: System Programming Author: Andreas Herrmann <herrmann.der.user@googlemail.com> Description ----------- +1) Processor TDP (Thermal design power) + +Given a fixed frequency and voltage, the power consumption of a +processor varies based on the workload being executed. Derated power +is the power consumed when running a specific application. Thermal +design power (TDP) is an example of derated power. + This driver permits reading of registers providing power information -of AMD Family 15h and 16h processors. +of AMD Family 15h and 16h processors via TDP algorithm. For AMD Family 15h and 16h processors the following power values can be calculated using different processor northbridge function @@ -37,3 +45,58 @@ This driver provides ProcessorPwrWatts and CurrPwrWatts: On multi-node processors the calculated value is for the entire package and not for a single node. Thus the driver creates sysfs attributes only for internal node0 of a multi-node processor. + +2) Accumulated Power Mechanism + +This driver also introduces an algorithm that should be used to +calculate the average power consumed by a processor during a +measurement interval Tm. The feature of accumulated power mechanism is +indicated by CPUID Fn8000_0007_EDX[12]. + +* Tsample: compute unit power accumulator sample period +* Tref: the PTSC counter period +* PTSC: performance timestamp counter +* N: the ratio of compute unit power accumulator sample period to the + PTSC period +* Jmax: max compute unit accumulated power which is indicated by + MaxCpuSwPwrAcc MSR C001007b +* Jx/Jy: compute unit accumulated power which is indicated by + CpuSwPwrAcc MSR C001007a +* Tx/Ty: the value of performance timestamp counter which is indicated + by CU_PTSC MSR C0010280 +* PwrCPUave: CPU average power + +i. Determine the ratio of Tsample to Tref by executing CPUID Fn8000_0007. + N = value of CPUID Fn8000_0007_ECX[CpuPwrSampleTimeRatio[15:0]]. + +ii. Read the full range of the cumulative energy value from the new +MSR MaxCpuSwPwrAcc. + Jmax = value returned. +iii. At time x, SW reads CpuSwPwrAcc MSR and samples the PTSC. + Jx = value read from CpuSwPwrAcc and Tx = value read from +PTSC. + +iv. At time y, SW reads CpuSwPwrAcc MSR and samples the PTSC. + Jy = value read from CpuSwPwrAcc and Ty = value read from +PTSC. + +v. Calculate the average power consumption for a compute unit over +time period (y-x). Unit of result is uWatt. + if (Jy < Jx) // Rollover has occurred + Jdelta = (Jy + Jmax) - Jx + else + Jdelta = Jy - Jx + PwrCPUave = N * Jdelta * 1000 / (Ty - Tx) + +This driver provides PwrCPUave and interval(default is 10 millisecond +and maximum is 1 second): +* power1_average (PwrCPUave) +* power1_average_interval (Interval) + +The power1_average_interval can be updated at /etc/sensors3.conf file +as below: + +chip "fam15h_power-*" + set power1_average_interval 0.01 + +Then save it with "sensors -s". diff --git a/Documentation/hwmon/it87 b/Documentation/hwmon/it87 index 733296d65449..fff6f6bf55bc 100644 --- a/Documentation/hwmon/it87 +++ b/Documentation/hwmon/it87 @@ -9,6 +9,9 @@ Supported chips: * IT8620E Prefix: 'it8620' Addresses scanned: from Super I/O config space (8 I/O ports) + * IT8628E + Prefix: 'it8628' + Addresses scanned: from Super I/O config space (8 I/O ports) Datasheet: Not publicly available * IT8705F Prefix: 'it87' @@ -114,8 +117,8 @@ motherboard models. Description ----------- -This driver implements support for the IT8603E, IT8620E, IT8623E, IT8705F, -IT8712F, IT8716F, IT8718F, IT8720F, IT8721F, IT8726F, IT8728F, IT8732F, +This driver implements support for the IT8603E, IT8620E, IT8623E, IT8628E, +IT8705F, IT8712F, IT8716F, IT8718F, IT8720F, IT8721F, IT8726F, IT8728F, IT8732F, IT8758E, IT8771E, IT8772E, IT8781F, IT8782F, IT8783E/F, IT8786E, IT8790E, and SiS950 chips. @@ -158,8 +161,8 @@ The IT8603E/IT8623E is a custom design, hardware monitoring part is similar to IT8728F. It only supports 3 fans, 16-bit fan mode, and the full speed mode of the fan is not supported (value 0 of pwmX_enable). -The IT8620E is another custom design, hardware monitoring part is similar to -IT8728F. It only supports 16-bit fan mode. +The IT8620E and IT8628E are custom designs, hardware monitoring part is similar +to IT8728F. It only supports 16-bit fan mode. Both chips support up to 6 fans. The IT8790E supports up to 3 fans. 16-bit fan mode is always enabled. @@ -187,8 +190,8 @@ of 0.016 volt. IT8603E, IT8721F/IT8758E and IT8728F can measure between 0 and 2.8 volts with a resolution of 0.0109 volt. The battery voltage in8 does not have limit registers. -On the IT8603E, IT8721F/IT8758E, IT8732F, IT8781F, IT8782F, and IT8783E/F, some -voltage inputs are internal and scaled inside the chip: +On the IT8603E, IT8620E, IT8628E, IT8721F/IT8758E, IT8732F, IT8781F, IT8782F, +and IT8783E/F, some voltage inputs are internal and scaled inside the chip: * in3 (optional) * in7 (optional for IT8781F, IT8782F, and IT8783E/F) * in8 (always) diff --git a/Documentation/hwmon/max31722 b/Documentation/hwmon/max31722 new file mode 100644 index 000000000000..090da84538c8 --- /dev/null +++ b/Documentation/hwmon/max31722 @@ -0,0 +1,34 @@ +Kernel driver max31722 +====================== + +Supported chips: + * Maxim Integrated MAX31722 + Prefix: 'max31722' + ACPI ID: MAX31722 + Addresses scanned: - + Datasheet: https://datasheets.maximintegrated.com/en/ds/MAX31722-MAX31723.pdf + * Maxim Integrated MAX31723 + Prefix: 'max31723' + ACPI ID: MAX31723 + Addresses scanned: - + Datasheet: https://datasheets.maximintegrated.com/en/ds/MAX31722-MAX31723.pdf + +Author: Tiberiu Breana <tiberiu.a.breana@intel.com> + +Description +----------- + +This driver adds support for the Maxim Integrated MAX31722/MAX31723 thermometers +and thermostats running over an SPI interface. + +Usage Notes +----------- + +This driver uses ACPI to auto-detect devices. See ACPI IDs in the above section. + +Sysfs entries +------------- + +The following attribute is supported: + +temp1_input Measured temperature. Read-only. diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt index c48f387109e8..d3240feef367 100644 --- a/Documentation/kernel-parameters.txt +++ b/Documentation/kernel-parameters.txt @@ -131,6 +131,7 @@ parameter is applicable: More X86-64 boot options can be found in Documentation/x86/x86_64/boot-options.txt . X86 Either 32-bit or 64-bit x86 (same as X86-32+X86-64) + X86_UV SGI UV support is enabled. XEN Xen support is enabled In addition, the following text indicates that the option: @@ -167,16 +168,18 @@ bytes respectively. Such letter suffixes can also be entirely omitted. acpi= [HW,ACPI,X86,ARM64] Advanced Configuration and Power Interface - Format: { force | off | strict | noirq | rsdt | + Format: { force | on | off | strict | noirq | rsdt | copy_dsdt } force -- enable ACPI if default was off + on -- enable ACPI but allow fallback to DT [arm64] off -- disable ACPI if default was on noirq -- do not use ACPI for IRQ routing strict -- Be less tolerant of platforms that are not strictly ACPI specification compliant. rsdt -- prefer RSDT over (default) XSDT copy_dsdt -- copy DSDT to memory - For ARM64, ONLY "acpi=off" or "acpi=force" are available + For ARM64, ONLY "acpi=off", "acpi=on" or "acpi=force" + are available See also Documentation/power/runtime_pm.txt, pci=noacpi @@ -544,6 +547,13 @@ bytes respectively. Such letter suffixes can also be entirely omitted. Format: <int> (must be >=0) Default: 64 + bau= [X86_UV] Enable the BAU on SGI UV. The default + behavior is to disable the BAU (i.e. bau=0). + Format: { "0" | "1" } + 0 - Disable the BAU. + 1 - Enable the BAU. + unset - Disable the BAU. + baycom_epp= [HW,AX25] Format: <io>,<mode> @@ -1663,6 +1673,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted. hwp_only Only load intel_pstate on systems which support hardware P state control (HWP) if available. + support_acpi_ppc + Enforce ACPI _PPC performance limits. If the Fixed ACPI + Description Table, specifies preferred power management + profile as "Enterprise Server" or "Performance Server", + then this feature is turned on by default. intremap= [X86-64, Intel-IOMMU] on enable Interrupt Remapping (default) @@ -3286,6 +3301,44 @@ bytes respectively. Such letter suffixes can also be entirely omitted. Lazy RCU callbacks are those which RCU can prove do nothing more than free memory. + rcuperf.gp_exp= [KNL] + Measure performance of expedited synchronous + grace-period primitives. + + rcuperf.holdoff= [KNL] + Set test-start holdoff period. The purpose of + this parameter is to delay the start of the + test until boot completes in order to avoid + interference. + + rcuperf.nreaders= [KNL] + Set number of RCU readers. The value -1 selects + N, where N is the number of CPUs. A value + "n" less than -1 selects N-n+1, where N is again + the number of CPUs. For example, -2 selects N + (the number of CPUs), -3 selects N+1, and so on. + A value of "n" less than or equal to -N selects + a single reader. + + rcuperf.nwriters= [KNL] + Set number of RCU writers. The values operate + the same as for rcuperf.nreaders. + N, where N is the number of CPUs + + rcuperf.perf_runnable= [BOOT] + Start rcuperf running at boot time. + + rcuperf.shutdown= [KNL] + Shut the system down after performance tests + complete. This is useful for hands-off automated + testing. + + rcuperf.perf_type= [KNL] + Specify the RCU implementation to test. + + rcuperf.verbose= [KNL] + Enable additional printk() statements. + rcutorture.cbflood_inter_holdoff= [KNL] Set holdoff time (jiffies) between successive callback-flood tests. diff --git a/Documentation/locking/lockdep-design.txt b/Documentation/locking/lockdep-design.txt index 5001280e9d82..9de1c158d44c 100644 --- a/Documentation/locking/lockdep-design.txt +++ b/Documentation/locking/lockdep-design.txt @@ -97,7 +97,7 @@ between any two lock-classes: <hardirq-safe> -> <hardirq-unsafe> <softirq-safe> -> <softirq-unsafe> -The first rule comes from the fact the a hardirq-safe lock could be +The first rule comes from the fact that a hardirq-safe lock could be taken by a hardirq context, interrupting a hardirq-unsafe lock - and thus could result in a lock inversion deadlock. Likewise, a softirq-safe lock could be taken by an softirq context, interrupting a softirq-unsafe @@ -220,7 +220,7 @@ calculated, which hash is unique for every lock chain. The hash value, when the chain is validated for the first time, is then put into a hash table, which hash-table can be checked in a lockfree manner. If the locking chain occurs again later on, the hash table tells us that we -dont have to validate the chain again. +don't have to validate the chain again. Troubleshooting: ---------------- diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index 3729cbe60e41..147ae8ec836f 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -4,8 +4,40 @@ By: David Howells <dhowells@redhat.com> Paul E. McKenney <paulmck@linux.vnet.ibm.com> + Will Deacon <will.deacon@arm.com> + Peter Zijlstra <peterz@infradead.org> -Contents: +========== +DISCLAIMER +========== + +This document is not a specification; it is intentionally (for the sake of +brevity) and unintentionally (due to being human) incomplete. This document is +meant as a guide to using the various memory barriers provided by Linux, but +in case of any doubt (and there are many) please ask. + +To repeat, this document is not a specification of what Linux expects from +hardware. + +The purpose of this document is twofold: + + (1) to specify the minimum functionality that one can rely on for any + particular barrier, and + + (2) to provide a guide as to how to use the barriers that are available. + +Note that an architecture can provide more than the minimum requirement +for any particular barrier, but if the architecure provides less than +that, that architecture is incorrect. + +Note also that it is possible that a barrier may be a no-op for an +architecture because the way that arch works renders an explicit barrier +unnecessary in that case. + + +======== +CONTENTS +======== (*) Abstract memory access model. @@ -31,15 +63,15 @@ Contents: (*) Implicit kernel memory barriers. - - Locking functions. + - Lock acquisition functions. - Interrupt disabling functions. - Sleep and wake-up functions. - Miscellaneous functions. - (*) Inter-CPU locking barrier effects. + (*) Inter-CPU acquiring barrier effects. - - Locks vs memory accesses. - - Locks vs I/O accesses. + - Acquires vs memory accesses. + - Acquires vs I/O accesses. (*) Where are memory barriers needed? @@ -61,6 +93,7 @@ Contents: (*) The things CPUs get up to. - And then there's the Alpha. + - Virtual Machine Guests. (*) Example uses. @@ -148,7 +181,7 @@ As a further example, consider this sequence of events: CPU 1 CPU 2 =============== =============== - { A == 1, B == 2, C = 3, P == &A, Q == &C } + { A == 1, B == 2, C == 3, P == &A, Q == &C } B = 4; Q = P; P = &B D = *Q; @@ -430,8 +463,9 @@ And a couple of implicit varieties: This acts as a one-way permeable barrier. It guarantees that all memory operations after the ACQUIRE operation will appear to happen after the ACQUIRE operation with respect to the other components of the system. - ACQUIRE operations include LOCK operations and smp_load_acquire() - operations. + ACQUIRE operations include LOCK operations and both smp_load_acquire() + and smp_cond_acquire() operations. The later builds the necessary ACQUIRE + semantics from relying on a control dependency and smp_rmb(). Memory operations that occur before an ACQUIRE operation may appear to happen after it completes. @@ -464,6 +498,11 @@ And a couple of implicit varieties: This means that ACQUIRE acts as a minimal "acquire" operation and RELEASE acts as a minimal "release" operation. +A subset of the atomic operations described in atomic_ops.txt have ACQUIRE +and RELEASE variants in addition to fully-ordered and relaxed (no barrier +semantics) definitions. For compound atomics performing both a load and a +store, ACQUIRE semantics apply only to the load and RELEASE semantics apply +only to the store portion of the operation. Memory barriers are only required where there's a possibility of interaction between two CPUs or between a CPU and a device. If it can be guaranteed that @@ -517,7 +556,7 @@ following sequence of events: CPU 1 CPU 2 =============== =============== - { A == 1, B == 2, C = 3, P == &A, Q == &C } + { A == 1, B == 2, C == 3, P == &A, Q == &C } B = 4; <write barrier> WRITE_ONCE(P, &B) @@ -544,7 +583,7 @@ between the address load and the data load: CPU 1 CPU 2 =============== =============== - { A == 1, B == 2, C = 3, P == &A, Q == &C } + { A == 1, B == 2, C == 3, P == &A, Q == &C } B = 4; <write barrier> WRITE_ONCE(P, &B); @@ -813,9 +852,10 @@ In summary: the same variable, then those stores must be ordered, either by preceding both of them with smp_mb() or by using smp_store_release() to carry out the stores. Please note that it is -not- sufficient - to use barrier() at beginning of each leg of the "if" statement, - as optimizing compilers do not necessarily respect barrier() - in this case. + to use barrier() at beginning of each leg of the "if" statement + because, as shown by the example above, optimizing compilers can + destroy the control dependency while respecting the letter of the + barrier() law. (*) Control dependencies require at least one run-time conditional between the prior load and the subsequent store, and this @@ -1731,15 +1771,15 @@ The Linux kernel has eight basic CPU memory barriers: All memory barriers except the data dependency barriers imply a compiler -barrier. Data dependencies do not impose any additional compiler ordering. +barrier. Data dependencies do not impose any additional compiler ordering. Aside: In the case of data dependencies, the compiler would be expected to issue the loads in the correct order (eg. `a[b]` would have to load the value of b before loading a[b]), however there is no guarantee in the C specification that the compiler may not speculate the value of b (eg. is equal to 1) and load a before b (eg. tmp = a[1]; if (b != 1) -tmp = a[b]; ). There is also the problem of a compiler reloading b after -having loaded a[b], thus having a newer copy of b than a[b]. A consensus +tmp = a[b]; ). There is also the problem of a compiler reloading b after +having loaded a[b], thus having a newer copy of b than a[b]. A consensus has not yet been reached about these problems, however the READ_ONCE() macro is a good place to start looking. @@ -1794,6 +1834,7 @@ There are some more advanced barrier functions: (*) lockless_dereference(); + This can be thought of as a pointer-fetch wrapper around the smp_read_barrier_depends() data-dependency barrier. @@ -1858,7 +1899,7 @@ This is a variation on the mandatory write barrier that causes writes to weakly ordered I/O regions to be partially ordered. Its effects may go beyond the CPU->Hardware interface and actually affect the hardware at some level. -See the subsection "Locks vs I/O accesses" for more information. +See the subsection "Acquires vs I/O accesses" for more information. =============================== @@ -1873,8 +1914,8 @@ provide more substantial guarantees, but these may not be relied upon outside of arch specific code. -ACQUIRING FUNCTIONS -------------------- +LOCK ACQUISITION FUNCTIONS +-------------------------- The Linux kernel has a number of locking constructs: @@ -1895,7 +1936,7 @@ for each construct. These operations all imply certain barriers: Memory operations issued before the ACQUIRE may be completed after the ACQUIRE operation has completed. An smp_mb__before_spinlock(), combined with a following ACQUIRE, orders prior stores against - subsequent loads and stores. Note that this is weaker than smp_mb()! + subsequent loads and stores. Note that this is weaker than smp_mb()! The smp_mb__before_spinlock() primitive is free on many architectures. (2) RELEASE operation implication: @@ -2090,9 +2131,9 @@ or: event_indicated = 1; wake_up_process(event_daemon); -A write memory barrier is implied by wake_up() and co. if and only if they wake -something up. The barrier occurs before the task state is cleared, and so sits -between the STORE to indicate the event and the STORE to set TASK_RUNNING: +A write memory barrier is implied by wake_up() and co. if and only if they +wake something up. The barrier occurs before the task state is cleared, and so +sits between the STORE to indicate the event and the STORE to set TASK_RUNNING: CPU 1 CPU 2 =============================== =============================== @@ -2206,7 +2247,7 @@ three CPUs; then should the following sequence of events occur: Then there is no guarantee as to what order CPU 3 will see the accesses to *A through *H occur in, other than the constraints imposed by the separate locks -on the separate CPUs. It might, for example, see: +on the separate CPUs. It might, for example, see: *E, ACQUIRE M, ACQUIRE Q, *G, *C, *F, *A, *B, RELEASE Q, *D, *H, RELEASE M @@ -2486,9 +2527,9 @@ The following operations are special locking primitives: clear_bit_unlock(); __clear_bit_unlock(); -These implement ACQUIRE-class and RELEASE-class operations. These should be used in -preference to other operations when implementing locking primitives, because -their implementations can be optimised on many architectures. +These implement ACQUIRE-class and RELEASE-class operations. These should be +used in preference to other operations when implementing locking primitives, +because their implementations can be optimised on many architectures. [!] Note that special memory barrier primitives are available for these situations because on some CPUs the atomic instructions used imply full memory @@ -2568,12 +2609,12 @@ explicit barriers are used. Normally this won't be a problem because the I/O accesses done inside such sections will include synchronous load operations on strictly ordered I/O -registers that form implicit I/O barriers. If this isn't sufficient then an +registers that form implicit I/O barriers. If this isn't sufficient then an mmiowb() may need to be used explicitly. A similar situation may occur between an interrupt routine and two routines -running on separate CPUs that communicate with each other. If such a case is +running on separate CPUs that communicate with each other. If such a case is likely, then interrupt-disabling locks should be used to guarantee ordering. @@ -2587,8 +2628,8 @@ functions: (*) inX(), outX(): These are intended to talk to I/O space rather than memory space, but - that's primarily a CPU-specific concept. The i386 and x86_64 processors do - indeed have special I/O space access cycles and instructions, but many + that's primarily a CPU-specific concept. The i386 and x86_64 processors + do indeed have special I/O space access cycles and instructions, but many CPUs don't have such a concept. The PCI bus, amongst others, defines an I/O space concept which - on such @@ -2610,7 +2651,7 @@ functions: Whether these are guaranteed to be fully ordered and uncombined with respect to each other on the issuing CPU depends on the characteristics - defined for the memory window through which they're accessing. On later + defined for the memory window through which they're accessing. On later i386 architecture machines, for example, this is controlled by way of the MTRR registers. @@ -2635,10 +2676,10 @@ functions: (*) readX_relaxed(), writeX_relaxed() These are similar to readX() and writeX(), but provide weaker memory - ordering guarantees. Specifically, they do not guarantee ordering with + ordering guarantees. Specifically, they do not guarantee ordering with respect to normal memory accesses (e.g. DMA buffers) nor do they guarantee - ordering with respect to LOCK or UNLOCK operations. If the latter is - required, an mmiowb() barrier can be used. Note that relaxed accesses to + ordering with respect to LOCK or UNLOCK operations. If the latter is + required, an mmiowb() barrier can be used. Note that relaxed accesses to the same peripheral are guaranteed to be ordered with respect to each other. @@ -3040,8 +3081,9 @@ The Alpha defines the Linux kernel's memory barrier model. See the subsection on "Cache Coherency" above. + VIRTUAL MACHINE GUESTS -------------------- +---------------------- Guests running within virtual machines might be affected by SMP effects even if the guest itself is compiled without SMP support. This is an artifact of @@ -3050,7 +3092,7 @@ barriers for this use-case would be possible but is often suboptimal. To handle this case optimally, low-level virt_mb() etc macros are available. These have the same effect as smp_mb() etc when SMP is enabled, but generate -identical code for SMP and non-SMP systems. For example, virtual machine guests +identical code for SMP and non-SMP systems. For example, virtual machine guests should use virt_mb() rather than smp_mb() when synchronizing against a (possibly SMP) host. @@ -3058,6 +3100,7 @@ These are equivalent to smp_mb() etc counterparts in all other respects, in particular, they do not control MMIO effects: to control MMIO effects, use mandatory barriers. + ============ EXAMPLE USES ============ diff --git a/Documentation/networking/checksum-offloads.txt b/Documentation/networking/checksum-offloads.txt index de2a327766a7..56e36861245f 100644 --- a/Documentation/networking/checksum-offloads.txt +++ b/Documentation/networking/checksum-offloads.txt @@ -69,18 +69,18 @@ LCO: Local Checksum Offload LCO is a technique for efficiently computing the outer checksum of an encapsulated datagram when the inner checksum is due to be offloaded. The ones-complement sum of a correctly checksummed TCP or UDP packet is - equal to the sum of the pseudo header, because everything else gets - 'cancelled out' by the checksum field. This is because the sum was + equal to the complement of the sum of the pseudo header, because everything + else gets 'cancelled out' by the checksum field. This is because the sum was complemented before being written to the checksum field. More generally, this holds in any case where the 'IP-style' ones complement checksum is used, and thus any checksum that TX Checksum Offload supports. That is, if we have set up TX Checksum Offload with a start/offset pair, we - know that _after the device has filled in that checksum_, the ones + know that after the device has filled in that checksum, the ones complement sum from csum_start to the end of the packet will be equal to - _whatever value we put in the checksum field beforehand_. This allows us - to compute the outer checksum without looking at the payload: we simply - stop summing when we get to csum_start, then add the 16-bit word at - (csum_start + csum_offset). + the complement of whatever value we put in the checksum field beforehand. + This allows us to compute the outer checksum without looking at the payload: + we simply stop summing when we get to csum_start, then add the complement of + the 16-bit word at (csum_start + csum_offset). Then, when the true inner checksum is filled in (either by hardware or by skb_checksum_help()), the outer checksum will become correct by virtue of the arithmetic. diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt index 57653a44b128..daabdd7ee543 100644 --- a/Documentation/sysctl/kernel.txt +++ b/Documentation/sysctl/kernel.txt @@ -60,6 +60,7 @@ show up in /proc/sys/kernel: - panic_on_warn - perf_cpu_time_max_percent - perf_event_paranoid +- perf_event_max_stack - pid_max - powersave-nap [ PPC only ] - printk @@ -645,7 +646,7 @@ allowed to execute. perf_event_paranoid: Controls use of the performance events system by unprivileged -users (without CAP_SYS_ADMIN). The default value is 1. +users (without CAP_SYS_ADMIN). The default value is 2. -1: Allow use of (almost) all events by all users >=0: Disallow raw tracepoint access by users without CAP_IOC_LOCK @@ -654,6 +655,19 @@ users (without CAP_SYS_ADMIN). The default value is 1. ============================================================== +perf_event_max_stack: + +Controls maximum number of stack frames to copy for (attr.sample_type & +PERF_SAMPLE_CALLCHAIN) configured events, for instance, when using +'perf record -g' or 'perf trace --call-graph fp'. + +This can only be done when no events are in use that have callchains +enabled, otherwise writing to this file will return -EBUSY. + +The default value is 127. + +============================================================== + pid_max: PID allocation wrap value. When the kernel's next PID value diff --git a/Documentation/trace/ftrace.txt b/Documentation/trace/ftrace.txt index f52f297cb406..9857606dd7b7 100644 --- a/Documentation/trace/ftrace.txt +++ b/Documentation/trace/ftrace.txt @@ -1562,12 +1562,12 @@ Doing the same with chrt -r 5 and function-trace set. <idle>-0 3dN.1 12us : menu_hrtimer_cancel <-tick_nohz_idle_exit <idle>-0 3dN.1 12us : ktime_get <-tick_nohz_idle_exit <idle>-0 3dN.1 12us : tick_do_update_jiffies64 <-tick_nohz_idle_exit - <idle>-0 3dN.1 13us : update_cpu_load_nohz <-tick_nohz_idle_exit - <idle>-0 3dN.1 13us : _raw_spin_lock <-update_cpu_load_nohz + <idle>-0 3dN.1 13us : cpu_load_update_nohz <-tick_nohz_idle_exit + <idle>-0 3dN.1 13us : _raw_spin_lock <-cpu_load_update_nohz <idle>-0 3dN.1 13us : add_preempt_count <-_raw_spin_lock - <idle>-0 3dN.2 13us : __update_cpu_load <-update_cpu_load_nohz - <idle>-0 3dN.2 14us : sched_avg_update <-__update_cpu_load - <idle>-0 3dN.2 14us : _raw_spin_unlock <-update_cpu_load_nohz + <idle>-0 3dN.2 13us : __cpu_load_update <-cpu_load_update_nohz + <idle>-0 3dN.2 14us : sched_avg_update <-__cpu_load_update + <idle>-0 3dN.2 14us : _raw_spin_unlock <-cpu_load_update_nohz <idle>-0 3dN.2 14us : sub_preempt_count <-_raw_spin_unlock <idle>-0 3dN.1 15us : calc_load_exit_idle <-tick_nohz_idle_exit <idle>-0 3dN.1 15us : touch_softlockup_watchdog <-tick_nohz_idle_exit diff --git a/Documentation/x86/pat.txt b/Documentation/x86/pat.txt index 54944c71b819..2a4ee6302122 100644 --- a/Documentation/x86/pat.txt +++ b/Documentation/x86/pat.txt @@ -196,3 +196,35 @@ Another, more verbose way of getting PAT related debug messages is with "debugpat" boot parameter. With this parameter, various debug messages are printed to dmesg log. +PAT Initialization +------------------ + +The following table describes how PAT is initialized under various +configurations. The PAT MSR must be updated by Linux in order to support WC +and WT attributes. Otherwise, the PAT MSR has the value programmed in it +by the firmware. Note, Xen enables WC attribute in the PAT MSR for guests. + + MTRR PAT Call Sequence PAT State PAT MSR + ========================================================= + E E MTRR -> PAT init Enabled OS + E D MTRR -> PAT init Disabled - + D E MTRR -> PAT disable Disabled BIOS + D D MTRR -> PAT disable Disabled - + - np/E PAT -> PAT disable Disabled BIOS + - np/D PAT -> PAT disable Disabled - + E !P/E MTRR -> PAT init Disabled BIOS + D !P/E MTRR -> PAT disable Disabled BIOS + !M !P/E MTRR stub -> PAT disable Disabled BIOS + + Legend + ------------------------------------------------ + E Feature enabled in CPU + D Feature disabled/unsupported in CPU + np "nopat" boot option specified + !P CONFIG_X86_PAT option unset + !M CONFIG_MTRR option unset + Enabled PAT state set to enabled + Disabled PAT state set to disabled + OS PAT initializes PAT MSR with OS setting + BIOS PAT keeps PAT MSR with BIOS setting + |
