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Diffstat (limited to 'Documentation/kernel-hacking/locking.rst')
| -rw-r--r-- | Documentation/kernel-hacking/locking.rst | 176 |
1 files changed, 88 insertions, 88 deletions
diff --git a/Documentation/kernel-hacking/locking.rst b/Documentation/kernel-hacking/locking.rst index a8518ac0d31d..6ed806e6061b 100644 --- a/Documentation/kernel-hacking/locking.rst +++ b/Documentation/kernel-hacking/locking.rst @@ -150,17 +150,17 @@ Locking Only In User Context If you have a data structure which is only ever accessed from user context, then you can use a simple mutex (``include/linux/mutex.h``) to protect it. This is the most trivial case: you initialize the mutex. -Then you can call :c:func:`mutex_lock_interruptible()` to grab the -mutex, and :c:func:`mutex_unlock()` to release it. There is also a -:c:func:`mutex_lock()`, which should be avoided, because it will +Then you can call mutex_lock_interruptible() to grab the +mutex, and mutex_unlock() to release it. There is also a +mutex_lock(), which should be avoided, because it will not return if a signal is received. Example: ``net/netfilter/nf_sockopt.c`` allows registration of new -:c:func:`setsockopt()` and :c:func:`getsockopt()` calls, with -:c:func:`nf_register_sockopt()`. Registration and de-registration +setsockopt() and getsockopt() calls, with +nf_register_sockopt(). Registration and de-registration are only done on module load and unload (and boot time, where there is no concurrency), and the list of registrations is only consulted for an -unknown :c:func:`setsockopt()` or :c:func:`getsockopt()` system +unknown setsockopt() or getsockopt() system call. The ``nf_sockopt_mutex`` is perfect to protect this, especially since the setsockopt and getsockopt calls may well sleep. @@ -170,19 +170,19 @@ Locking Between User Context and Softirqs If a softirq shares data with user context, you have two problems. Firstly, the current user context can be interrupted by a softirq, and secondly, the critical region could be entered from another CPU. This is -where :c:func:`spin_lock_bh()` (``include/linux/spinlock.h``) is +where spin_lock_bh() (``include/linux/spinlock.h``) is used. It disables softirqs on that CPU, then grabs the lock. -:c:func:`spin_unlock_bh()` does the reverse. (The '_bh' suffix is +spin_unlock_bh() does the reverse. (The '_bh' suffix is a historical reference to "Bottom Halves", the old name for software interrupts. It should really be called spin_lock_softirq()' in a perfect world). -Note that you can also use :c:func:`spin_lock_irq()` or -:c:func:`spin_lock_irqsave()` here, which stop hardware interrupts +Note that you can also use spin_lock_irq() or +spin_lock_irqsave() here, which stop hardware interrupts as well: see `Hard IRQ Context <#hard-irq-context>`__. This works perfectly for UP as well: the spin lock vanishes, and this -macro simply becomes :c:func:`local_bh_disable()` +macro simply becomes local_bh_disable() (``include/linux/interrupt.h``), which protects you from the softirq being run. @@ -216,8 +216,8 @@ Different Tasklets/Timers ~~~~~~~~~~~~~~~~~~~~~~~~~ If another tasklet/timer wants to share data with your tasklet or timer -, you will both need to use :c:func:`spin_lock()` and -:c:func:`spin_unlock()` calls. :c:func:`spin_lock_bh()` is +, you will both need to use spin_lock() and +spin_unlock() calls. spin_lock_bh() is unnecessary here, as you are already in a tasklet, and none will be run on the same CPU. @@ -234,14 +234,14 @@ The same softirq can run on the other CPUs: you can use a per-CPU array going so far as to use a softirq, you probably care about scalable performance enough to justify the extra complexity. -You'll need to use :c:func:`spin_lock()` and -:c:func:`spin_unlock()` for shared data. +You'll need to use spin_lock() and +spin_unlock() for shared data. Different Softirqs ~~~~~~~~~~~~~~~~~~ -You'll need to use :c:func:`spin_lock()` and -:c:func:`spin_unlock()` for shared data, whether it be a timer, +You'll need to use spin_lock() and +spin_unlock() for shared data, whether it be a timer, tasklet, different softirq or the same or another softirq: any of them could be running on a different CPU. @@ -259,38 +259,38 @@ If a hardware irq handler shares data with a softirq, you have two concerns. Firstly, the softirq processing can be interrupted by a hardware interrupt, and secondly, the critical region could be entered by a hardware interrupt on another CPU. This is where -:c:func:`spin_lock_irq()` is used. It is defined to disable +spin_lock_irq() is used. It is defined to disable interrupts on that cpu, then grab the lock. -:c:func:`spin_unlock_irq()` does the reverse. +spin_unlock_irq() does the reverse. -The irq handler does not to use :c:func:`spin_lock_irq()`, because +The irq handler does not need to use spin_lock_irq(), because the softirq cannot run while the irq handler is running: it can use -:c:func:`spin_lock()`, which is slightly faster. The only exception +spin_lock(), which is slightly faster. The only exception would be if a different hardware irq handler uses the same lock: -:c:func:`spin_lock_irq()` will stop that from interrupting us. +spin_lock_irq() will stop that from interrupting us. This works perfectly for UP as well: the spin lock vanishes, and this -macro simply becomes :c:func:`local_irq_disable()` +macro simply becomes local_irq_disable() (``include/asm/smp.h``), which protects you from the softirq/tasklet/BH being run. -:c:func:`spin_lock_irqsave()` (``include/linux/spinlock.h``) is a +spin_lock_irqsave() (``include/linux/spinlock.h``) is a variant which saves whether interrupts were on or off in a flags word, -which is passed to :c:func:`spin_unlock_irqrestore()`. This means +which is passed to spin_unlock_irqrestore(). This means that the same code can be used inside an hard irq handler (where interrupts are already off) and in softirqs (where the irq disabling is required). Note that softirqs (and hence tasklets and timers) are run on return -from hardware interrupts, so :c:func:`spin_lock_irq()` also stops -these. In that sense, :c:func:`spin_lock_irqsave()` is the most +from hardware interrupts, so spin_lock_irq() also stops +these. In that sense, spin_lock_irqsave() is the most general and powerful locking function. Locking Between Two Hard IRQ Handlers ------------------------------------- It is rare to have to share data between two IRQ handlers, but if you -do, :c:func:`spin_lock_irqsave()` should be used: it is +do, spin_lock_irqsave() should be used: it is architecture-specific whether all interrupts are disabled inside irq handlers themselves. @@ -304,11 +304,11 @@ Pete Zaitcev gives the following summary: (``copy_from_user*(`` or ``kmalloc(x,GFP_KERNEL)``). - Otherwise (== data can be touched in an interrupt), use - :c:func:`spin_lock_irqsave()` and - :c:func:`spin_unlock_irqrestore()`. + spin_lock_irqsave() and + spin_unlock_irqrestore(). - Avoid holding spinlock for more than 5 lines of code and across any - function call (except accessors like :c:func:`readb()`). + function call (except accessors like readb()). Table of Minimum Requirements ----------------------------- @@ -320,7 +320,7 @@ particular thread can only run on one CPU at a time, but if it needs shares data with another thread, locking is required). Remember the advice above: you can always use -:c:func:`spin_lock_irqsave()`, which is a superset of all other +spin_lock_irqsave(), which is a superset of all other spinlock primitives. ============== ============= ============= ========= ========= ========= ========= ======= ======= ============== ============== @@ -363,13 +363,13 @@ They can be used if you need no access to the data protected with the lock when some other thread is holding the lock. You should acquire the lock later if you then need access to the data protected with the lock. -:c:func:`spin_trylock()` does not spin but returns non-zero if it +spin_trylock() does not spin but returns non-zero if it acquires the spinlock on the first try or 0 if not. This function can be -used in all contexts like :c:func:`spin_lock()`: you must have +used in all contexts like spin_lock(): you must have disabled the contexts that might interrupt you and acquire the spin lock. -:c:func:`mutex_trylock()` does not suspend your task but returns +mutex_trylock() does not suspend your task but returns non-zero if it could lock the mutex on the first try or 0 if not. This function cannot be safely used in hardware or software interrupt contexts despite not sleeping. @@ -490,14 +490,14 @@ easy, since we copy the data for the user, and never let them access the objects directly. There is a slight (and common) optimization here: in -:c:func:`cache_add()` we set up the fields of the object before +cache_add() we set up the fields of the object before grabbing the lock. This is safe, as no-one else can access it until we put it in cache. Accessing From Interrupt Context -------------------------------- -Now consider the case where :c:func:`cache_find()` can be called +Now consider the case where cache_find() can be called from interrupt context: either a hardware interrupt or a softirq. An example would be a timer which deletes object from the cache. @@ -566,16 +566,16 @@ which are taken away, and the ``+`` are lines which are added. return ret; } -Note that the :c:func:`spin_lock_irqsave()` will turn off +Note that the spin_lock_irqsave() will turn off interrupts if they are on, otherwise does nothing (if we are already in an interrupt handler), hence these functions are safe to call from any context. -Unfortunately, :c:func:`cache_add()` calls :c:func:`kmalloc()` +Unfortunately, cache_add() calls kmalloc() with the ``GFP_KERNEL`` flag, which is only legal in user context. I -have assumed that :c:func:`cache_add()` is still only called in +have assumed that cache_add() is still only called in user context, otherwise this should become a parameter to -:c:func:`cache_add()`. +cache_add(). Exposing Objects Outside This File ---------------------------------- @@ -592,7 +592,7 @@ This makes locking trickier, as it is no longer all in one place. The second problem is the lifetime problem: if another structure keeps a pointer to an object, it presumably expects that pointer to remain valid. Unfortunately, this is only guaranteed while you hold the lock, -otherwise someone might call :c:func:`cache_delete()` and even +otherwise someone might call cache_delete() and even worse, add another object, re-using the same address. As there is only one lock, you can't hold it forever: no-one else would @@ -693,8 +693,8 @@ Here is the code:: We encapsulate the reference counting in the standard 'get' and 'put' functions. Now we can return the object itself from -:c:func:`cache_find()` which has the advantage that the user can -now sleep holding the object (eg. to :c:func:`copy_to_user()` to +cache_find() which has the advantage that the user can +now sleep holding the object (eg. to copy_to_user() to name to userspace). The other point to note is that I said a reference should be held for @@ -710,7 +710,7 @@ number of atomic operations defined in ``include/asm/atomic.h``: these are guaranteed to be seen atomically from all CPUs in the system, so no lock is required. In this case, it is simpler than using spinlocks, although for anything non-trivial using spinlocks is clearer. The -:c:func:`atomic_inc()` and :c:func:`atomic_dec_and_test()` +atomic_inc() and atomic_dec_and_test() are used instead of the standard increment and decrement operators, and the lock is no longer used to protect the reference count itself. @@ -802,7 +802,7 @@ name to change, there are three possibilities: - You can make ``cache_lock`` non-static, and tell people to grab that lock before changing the name in any object. -- You can provide a :c:func:`cache_obj_rename()` which grabs this +- You can provide a cache_obj_rename() which grabs this lock and changes the name for the caller, and tell everyone to use that function. @@ -861,11 +861,11 @@ Note that I decide that the popularity count should be protected by the ``cache_lock`` rather than the per-object lock: this is because it (like the :c:type:`struct list_head <list_head>` inside the object) is logically part of the infrastructure. This way, I don't need to grab -the lock of every object in :c:func:`__cache_add()` when seeking +the lock of every object in __cache_add() when seeking the least popular. I also decided that the id member is unchangeable, so I don't need to -grab each object lock in :c:func:`__cache_find()` to examine the +grab each object lock in __cache_find() to examine the id: the object lock is only used by a caller who wants to read or write the name field. @@ -887,7 +887,7 @@ trivial to diagnose: not a stay-up-five-nights-talk-to-fluffy-code-bunnies kind of problem. For a slightly more complex case, imagine you have a region shared by a -softirq and user context. If you use a :c:func:`spin_lock()` call +softirq and user context. If you use a spin_lock() call to protect it, it is possible that the user context will be interrupted by the softirq while it holds the lock, and the softirq will then spin forever trying to get the same lock. @@ -985,12 +985,12 @@ you might do the following:: Sooner or later, this will crash on SMP, because a timer can have just -gone off before the :c:func:`spin_lock_bh()`, and it will only get -the lock after we :c:func:`spin_unlock_bh()`, and then try to free +gone off before the spin_lock_bh(), and it will only get +the lock after we spin_unlock_bh(), and then try to free the element (which has already been freed!). This can be avoided by checking the result of -:c:func:`del_timer()`: if it returns 1, the timer has been deleted. +del_timer(): if it returns 1, the timer has been deleted. If 0, it means (in this case) that it is currently running, so we can do:: @@ -1012,9 +1012,9 @@ do:: Another common problem is deleting timers which restart themselves (by -calling :c:func:`add_timer()` at the end of their timer function). +calling add_timer() at the end of their timer function). Because this is a fairly common case which is prone to races, you should -use :c:func:`del_timer_sync()` (``include/linux/timer.h``) to +use del_timer_sync() (``include/linux/timer.h``) to handle this case. It returns the number of times the timer had to be deleted before we finally stopped it from adding itself back in. @@ -1086,7 +1086,7 @@ adding ``new`` to a single linked list called ``list``:: list->next = new; -The :c:func:`wmb()` is a write memory barrier. It ensures that the +The wmb() is a write memory barrier. It ensures that the first operation (setting the new element's ``next`` pointer) is complete and will be seen by all CPUs, before the second operation is (putting the new element into the list). This is important, since modern @@ -1097,7 +1097,7 @@ rest of the list. Fortunately, there is a function to do this for standard :c:type:`struct list_head <list_head>` lists: -:c:func:`list_add_rcu()` (``include/linux/list.h``). +list_add_rcu() (``include/linux/list.h``). Removing an element from the list is even simpler: we replace the pointer to the old element with a pointer to its successor, and readers @@ -1108,7 +1108,7 @@ will either see it, or skip over it. list->next = old->next; -There is :c:func:`list_del_rcu()` (``include/linux/list.h``) which +There is list_del_rcu() (``include/linux/list.h``) which does this (the normal version poisons the old object, which we don't want). @@ -1116,9 +1116,9 @@ The reader must also be careful: some CPUs can look through the ``next`` pointer to start reading the contents of the next element early, but don't realize that the pre-fetched contents is wrong when the ``next`` pointer changes underneath them. Once again, there is a -:c:func:`list_for_each_entry_rcu()` (``include/linux/list.h``) +list_for_each_entry_rcu() (``include/linux/list.h``) to help you. Of course, writers can just use -:c:func:`list_for_each_entry()`, since there cannot be two +list_for_each_entry(), since there cannot be two simultaneous writers. Our final dilemma is this: when can we actually destroy the removed @@ -1127,14 +1127,14 @@ the list right now: if we free this element and the ``next`` pointer changes, the reader will jump off into garbage and crash. We need to wait until we know that all the readers who were traversing the list when we deleted the element are finished. We use -:c:func:`call_rcu()` to register a callback which will actually +call_rcu() to register a callback which will actually destroy the object once all pre-existing readers are finished. -Alternatively, :c:func:`synchronize_rcu()` may be used to block +Alternatively, synchronize_rcu() may be used to block until all pre-existing are finished. But how does Read Copy Update know when the readers are finished? The method is this: firstly, the readers always traverse the list inside -:c:func:`rcu_read_lock()`/:c:func:`rcu_read_unlock()` pairs: +rcu_read_lock()/rcu_read_unlock() pairs: these simply disable preemption so the reader won't go to sleep while reading the list. @@ -1223,12 +1223,12 @@ this is the fundamental idea. } Note that the reader will alter the popularity member in -:c:func:`__cache_find()`, and now it doesn't hold a lock. One +__cache_find(), and now it doesn't hold a lock. One solution would be to make it an ``atomic_t``, but for this usage, we don't really care about races: an approximate result is good enough, so I didn't change it. -The result is that :c:func:`cache_find()` requires no +The result is that cache_find() requires no synchronization with any other functions, so is almost as fast on SMP as it would be on UP. @@ -1240,9 +1240,9 @@ and put the reference count. Now, because the 'read lock' in RCU is simply disabling preemption, a caller which always has preemption disabled between calling -:c:func:`cache_find()` and :c:func:`object_put()` does not +cache_find() and object_put() does not need to actually get and put the reference count: we could expose -:c:func:`__cache_find()` by making it non-static, and such +__cache_find() by making it non-static, and such callers could simply call that. The benefit here is that the reference count is not written to: the @@ -1260,11 +1260,11 @@ counter. Nice and simple. If that was too slow (it's usually not, but if you've got a really big machine to test on and can show that it is), you could instead use a counter for each CPU, then none of them need an exclusive lock. See -:c:func:`DEFINE_PER_CPU()`, :c:func:`get_cpu_var()` and -:c:func:`put_cpu_var()` (``include/linux/percpu.h``). +DEFINE_PER_CPU(), get_cpu_var() and +put_cpu_var() (``include/linux/percpu.h``). Of particular use for simple per-cpu counters is the ``local_t`` type, -and the :c:func:`cpu_local_inc()` and related functions, which are +and the cpu_local_inc() and related functions, which are more efficient than simple code on some architectures (``include/asm/local.h``). @@ -1289,10 +1289,10 @@ irq handler doesn't use a lock, and all other accesses are done as so:: enable_irq(irq); spin_unlock(&lock); -The :c:func:`disable_irq()` prevents the irq handler from running +The disable_irq() prevents the irq handler from running (and waits for it to finish if it's currently running on other CPUs). The spinlock prevents any other accesses happening at the same time. -Naturally, this is slower than just a :c:func:`spin_lock_irq()` +Naturally, this is slower than just a spin_lock_irq() call, so it only makes sense if this type of access happens extremely rarely. @@ -1315,22 +1315,22 @@ from user context, and can sleep. - Accesses to userspace: - - :c:func:`copy_from_user()` + - copy_from_user() - - :c:func:`copy_to_user()` + - copy_to_user() - - :c:func:`get_user()` + - get_user() - - :c:func:`put_user()` + - put_user() -- :c:func:`kmalloc(GFP_KERNEL) <kmalloc>` +- kmalloc(GP_KERNEL) <kmalloc>` -- :c:func:`mutex_lock_interruptible()` and - :c:func:`mutex_lock()` +- mutex_lock_interruptible() and + mutex_lock() - There is a :c:func:`mutex_trylock()` which does not sleep. + There is a mutex_trylock() which does not sleep. Still, it must not be used inside interrupt context since its - implementation is not safe for that. :c:func:`mutex_unlock()` + implementation is not safe for that. mutex_unlock() will also never sleep. It cannot be used in interrupt context either since a mutex must be released by the same task that acquired it. @@ -1340,11 +1340,11 @@ Some Functions Which Don't Sleep Some functions are safe to call from any context, or holding almost any lock. -- :c:func:`printk()` +- printk() -- :c:func:`kfree()` +- kfree() -- :c:func:`add_timer()` and :c:func:`del_timer()` +- add_timer() and del_timer() Mutex API reference =================== @@ -1400,26 +1400,26 @@ preemption bh Bottom Half: for historical reasons, functions with '_bh' in them often - now refer to any software interrupt, e.g. :c:func:`spin_lock_bh()` + now refer to any software interrupt, e.g. spin_lock_bh() blocks any software interrupt on the current CPU. Bottom halves are deprecated, and will eventually be replaced by tasklets. Only one bottom half will be running at any time. Hardware Interrupt / Hardware IRQ - Hardware interrupt request. :c:func:`in_irq()` returns true in a + Hardware interrupt request. in_irq() returns true in a hardware interrupt handler. Interrupt Context Not user context: processing a hardware irq or software irq. Indicated - by the :c:func:`in_interrupt()` macro returning true. + by the in_interrupt() macro returning true. SMP Symmetric Multi-Processor: kernels compiled for multiple-CPU machines. (``CONFIG_SMP=y``). Software Interrupt / softirq - Software interrupt handler. :c:func:`in_irq()` returns false; - :c:func:`in_softirq()` returns true. Tasklets and softirqs both + Software interrupt handler. in_irq() returns false; + in_softirq() returns true. Tasklets and softirqs both fall into the category of 'software interrupts'. Strictly speaking a softirq is one of up to 32 enumerated software |