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 | 22 +++++++++++++++++++ 1 file changed, 22 insertions(+) diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst index 16c64a2eff93..0a4148b9f743 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.rst +++ b/Documentation/RCU/Design/Requirements/Requirements.rst @@ -1940,6 +1940,28 @@ 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 bookkeeping of every CPU's hotplug state, independent of +the various CPU masks and by reporting quiescent states at explicit points. It +may come across as a surprise, but the force quiescent state loop (FQS) does +not report quiescent states for offline CPUs and is not required to. + +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 a report +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 ~~~~~~~~~~~~~~~~~ -- 2.28.0.236.gb10cc79966-goog
