linux-kernel - [PATCH v2 3/4] locking/rwsem: Clarify that RWSEM_READER

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Date: Thu, 22 Feb 2024 10:05:39 -0500
From: Waiman Long <longman@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Will Deacon <will@...nel.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org,
	George Stark <gnstark@...utedevices.com>,
	Waiman Long <longman@...hat.com>
Subject: [PATCH v2 3/4] locking/rwsem: Clarify that RWSEM_READER_OWNED is just a hint

Clarify in the comments that the RWSEM_READER_OWNED bit in the owner
field is just a hint, not an authoritative state of the rwsem.

Signed-off-by: Waiman Long <longman@...hat.com>
Reviewed-by: Boqun Feng <boqun.feng@...il.com>
---
 kernel/locking/rwsem.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 2340b6d90ec6..c6d17aee4209 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -35,7 +35,7 @@
 /*
  * The least significant 2 bits of the owner value has the following
  * meanings when set.
- *  - Bit 0: RWSEM_READER_OWNED - The rwsem is owned by readers
+ *  - Bit 0: RWSEM_READER_OWNED - rwsem may be owned by readers (just a hint)
  *  - Bit 1: RWSEM_NONSPINNABLE - Cannot spin on a reader-owned lock
  *
  * When the rwsem is reader-owned and a spinning writer has timed out,
@@ -1002,8 +1002,8 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int stat
 
 	/*
 	 * To prevent a constant stream of readers from starving a sleeping
-	 * waiter, don't attempt optimistic lock stealing if the lock is
-	 * currently owned by readers.
+	 * writer, don't attempt optimistic lock stealing if the lock is
+	 * very likely owned by readers.
 	 */
 	if ((atomic_long_read(&sem->owner) & RWSEM_READER_OWNED) &&
 	    (rcnt > 1) && !(count & RWSEM_WRITER_LOCKED))
-- 
2.39.3