linux-kernel - [PATCH rcu 1/7] doc: Emphasize the need for explicit RCU read-side markers

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Message-Id: <20220831180625.2692892-1-paulmck@kernel.org>
Date:   Wed, 31 Aug 2022 11:06:19 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com,
        rostedt@...dmis.org, "Paul E. McKenney" <paulmck@...nel.org>
Subject: [PATCH rcu 1/7] doc: Emphasize the need for explicit RCU read-side markers

This commit updates checklist.rst to emphasize the need for explicit
markers for RCU read-side critical sections.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 Documentation/RCU/checklist.rst | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/Documentation/RCU/checklist.rst b/Documentation/RCU/checklist.rst
index 42cc5d891bd26..5eedef027d922 100644
--- a/Documentation/RCU/checklist.rst
+++ b/Documentation/RCU/checklist.rst
@@ -66,8 +66,13 @@ over a rather long period of time, but improvements are always welcome!
 	As a rough rule of thumb, any dereference of an RCU-protected
 	pointer must be covered by rcu_read_lock(), rcu_read_lock_bh(),
 	rcu_read_lock_sched(), or by the appropriate update-side lock.
-	Disabling of preemption can serve as rcu_read_lock_sched(), but
-	is less readable and prevents lockdep from detecting locking issues.
+	Explicit disabling of preemption (preempt_disable(), for example)
+	can serve as rcu_read_lock_sched(), but is less readable and
+	prevents lockdep from detecting locking issues.
+
+	Please not that you *cannot* rely on code known to be built
+	only in non-preemptible kernels.  Such code can and will break,
+	especially in kernels built with CONFIG_PREEMPT_COUNT=y.
 
 	Letting RCU-protected pointers "leak" out of an RCU read-side
 	critical section is every bit as bad as letting them leak out
-- 
2.31.1.189.g2e36527f23