[PATCH v4 5/5] docs: Update RCU's hotplug requirements with a bit about design

["Joel Fernandes (Google)" <joel@joelfernandes.org>]

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@joelfernandes.org>
---
 .../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