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| -rw-r--r-- | Documentation/RCU/Design/Requirements/Requirements.rst | 49 |
1 files changed, 39 insertions, 10 deletions
diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst index 1ae79a10a8de..8807985a9c35 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.rst +++ b/Documentation/RCU/Design/Requirements/Requirements.rst @@ -1929,16 +1929,45 @@ 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 synchronous -grace-period operations such as ``synchronize_rcu()`` and -``synchronize_rcu_expedited()``. - -However, all-callback-wait operations such as ``rcu_barrier()`` 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. Furthermore, ``rcu_barrier()`` blocks CPU-hotplug operations -during its execution, which results in another type of deadlock when -invoked from a CPU-hotplug notifier. +grace-period operations such as (``synchronize_rcu()`` and +``synchronize_rcu_expedited()``). However, these synchronous operations +do block and therefore cannot be invoked from notifiers that execute via +``stop_machine()``, specifically those between the ``CPUHP_AP_OFFLINE`` +and ``CPUHP_AP_ONLINE`` states. + +In addition, all-callback-wait operations such as ``rcu_barrier()`` may +not be invoked from any CPU-hotplug notifier. This restriction is 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. Furthermore, +``rcu_barrier()`` blocks CPU-hotplug operations during its execution, +which results in another type of deadlock when invoked from a CPU-hotplug +notifier. + +Finally, RCU must avoid deadlocks due to interaction between hotplug, +timers and grace period processing. It does so by maintaining its own set +of books that duplicate the centrally maintained ``cpu_online_mask``, +and also by reporting quiescent states explicitly when a CPU goes +offline. This explicit reporting of quiescent states avoids any need +for the force-quiescent-state loop (FQS) to report quiescent states for +offline CPUs. However, as a debugging measure, the FQS loop does splat +if offline CPUs block an RCU grace period for too long. + +An offline CPU's quiescent state will be reported either: +1. As the CPU goes offline using RCU's hotplug notifier (``rcu_report_dead()``). +2. When grace period initialization (``rcu_gp_init()``) detects a + race either with CPU offlining or with a task unblocking on a leaf + ``rcu_node`` structure whose CPUs are all offline. + +The CPU-online path (``rcu_cpu_starting()``) should never need to report +a quiescent state for an offline CPU. However, as a debugging measure, +it does emit a warning if a quiescent state was not already reported +for that CPU. + +During the checking/modification of RCU's hotplug bookkeeping, the +corresponding CPU's leaf node lock is held. This avoids race conditions +between RCU's hotplug notifier hooks, the grace period initialization +code, and the FQS loop, all of which refer to or modify this bookkeeping. Scheduler and RCU ~~~~~~~~~~~~~~~~~ |