linux-kernel - [PATCH 2/8] doc: Make whatisRCU.rst note that spinlocks are RCU readers

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Date: Thu, 25 Jan 2024 19:58:05 -0800
From: Boqun Feng <boqun.feng@...il.com>
To: linux-kernel@...r.kernel.org,
	rcu@...r.kernel.org,
	linux-doc@...r.kernel.org
Cc: Jonathan Corbet <corbet@....net>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Neeraj Upadhyay <neeraj.iitr10@...il.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Josh Triplett <josh@...htriplett.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>
Subject: [PATCH 2/8] doc: Make whatisRCU.rst note that spinlocks are RCU readers

From: "Paul E. McKenney" <paulmck@...nel.org>

In kernels built with CONFIG_PREEMPT_RT=n, spinlock critical sections
are RCU readers because they disable preemption.  However, they are also
RCU readers in CONFIG_PREEMPT_RT=y because in that case the locking
primitives contain rcu_read_lock() and rcu_read_unlock().  Therefore,
upgrade whatisRCU.rst to document this non-obvious case.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 Documentation/RCU/whatisRCU.rst | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/Documentation/RCU/whatisRCU.rst b/Documentation/RCU/whatisRCU.rst
index 60ce02475142..246ce0d0b4d1 100644
--- a/Documentation/RCU/whatisRCU.rst
+++ b/Documentation/RCU/whatisRCU.rst
@@ -172,14 +172,25 @@ rcu_read_lock()
 	critical section.  Reference counts may be used in conjunction
 	with RCU to maintain longer-term references to data structures.
 
+	Note that anything that disables bottom halves, preemption,
+	or interrupts also enters an RCU read-side critical section.
+	Acquiring a spinlock also enters an RCU read-side critical
+	sections, even for spinlocks that do not disable preemption,
+	as is the case in kernels built with CONFIG_PREEMPT_RT=y.
+	Sleeplocks do *not* enter RCU read-side critical sections.
+
 rcu_read_unlock()
 ^^^^^^^^^^^^^^^^^
 	void rcu_read_unlock(void);
 
 	This temporal primitives is used by a reader to inform the
 	reclaimer that the reader is exiting an RCU read-side critical
-	section.  Note that RCU read-side critical sections may be nested
-	and/or overlapping.
+	section.  Anything that enables bottom halves, preemption,
+	or interrupts also exits an RCU read-side critical section.
+	Releasing a spinlock also exits an RCU read-side critical section.
+
+	Note that RCU read-side critical sections may be nested and/or
+	overlapping.
 
 synchronize_rcu()
 ^^^^^^^^^^^^^^^^^
-- 
2.43.0