[tip: core/rcu] docs: Update RCU's hotplug requirements with a bit about design

[tip-bot2 for Joel Fernandes (Google)]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     a043260740d5d6ec5be59c3fb595c719890a0b0b
Gitweb:        https://git.kernel.org/tip/a043260740d5d6ec5be59c3fb595c719890a0b0b
Author:        Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
AuthorDate:    Tue, 29 Sep 2020 15:29:28 -04:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Fri, 06 Nov 2020 17:02:43 -08:00

docs: Update RCU's hotplug requirements with a bit about design

The rcu_barrier() section of the "Hotplug CPU" section discusses
deadlocks, however the description of deadlocks other than those involving
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>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 Documentation/RCU/Design/Requirements/Requirements.rst | 49 +++++++--
 1 file changed, 39 insertions(+), 10 deletions(-)

diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst
index 1ae79a1..8807985 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
 ~~~~~~~~~~~~~~~~~