RCU's hotplug design will help understand the requirements an RCU implementation needs to fullfill, such as dead-lock avoidance. The rcu_barrier() section of the "Hotplug CPU" section already talks about deadlocks, however the description of what else can deadlock other than rcu_barrier is rather incomplete. This commit therefore continues the section by describing how RCU's design handles CPU hotplug in a deadlock-free way. Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> --- .../RCU/Design/Requirements/Requirements.rst | 30 +++++++++++++++++-- 1 file changed, 28 insertions(+), 2 deletions(-) diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst index 1ae79a10a8de..e0413aa989dd 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.rst +++ b/Documentation/RCU/Design/Requirements/Requirements.rst @@ -1929,8 +1929,10 @@ 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()``. +grace-period operations such as. However, the synchronous variants +(``synchronize_rcu()`` and ``synchronize_rcu_expedited()``) should not +from notifiers that execute via ``stop_machine()`` -- specifically those +between the ``CPUHP_AP_OFFLINE`` and ``CPUHP_AP_ONLINE`` states. However, all-callback-wait operations such as ``rcu_barrier()`` are also not supported, due to the fact that there are phases of CPU-hotplug @@ -1940,6 +1942,30 @@ 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. +Also, RCU's implementation avoids serious deadlocks which could occur due to +interaction between hotplug, timers and grace period processing. It does so by +maintaining its own books of every CPU's hotplug state, independent of +the existing general-purpose CPU masks and by reporting quiescent states +explictly when an online CPU is going down. Due to this design, the force +quiescent state loop (FQS) is not required to report quiescent states for +offline CPUs, like it does for idle CPUs, but it does splat if offline CPUs are +stalling the RCU grace period for too long. + +For an offline CPU, the quiescent state will be reported in either of: +1. During CPU offlining, using RCU's hotplug notifier (``rcu_report_dead()``). +2. During grace period initialization (``rcu_gp_init()``) if it detected a race + with CPU offlining, or a race with a task unblocking on a node which + previously had all of its CPUs offlined. + +The CPU onlining path (``rcu_cpu_starting()``) does not need to report a +quiescent state for an offline CPU; in fact it would trigger 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, grace period initialization code and the FQS loop +which can concurrently refer to or modify the bookkeeping. + Scheduler and RCU ~~~~~~~~~~~~~~~~~ -- 2.28.0.709.gb0816b6eb0-goog
