linux-kernel - [tip: locking/core] locking/rwsem: Clarify that RWSEM_READER

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Date: Wed, 28 Feb 2024 12:53:21 -0000
From: "tip-bot2 for Waiman Long" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Waiman Long <longman@...hat.com>, Ingo Molnar <mingo@...nel.org>,
 Boqun Feng <boqun.feng@...il.com>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: locking/core] locking/rwsem: Clarify that RWSEM_READER_OWNED is
 just a hint

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

Commit-ID:     d566c78659eccf085f905fd266fc461de92eaa8f
Gitweb:        https://git.kernel.org/tip/d566c78659eccf085f905fd266fc461de92eaa8f
Author:        Waiman Long <longman@...hat.com>
AuthorDate:    Thu, 22 Feb 2024 10:05:39 -05:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Wed, 28 Feb 2024 13:08:38 +01:00

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>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Reviewed-by: Boqun Feng <boqun.feng@...il.com>
Link: https://lore.kernel.org/r/20240222150540.79981-4-longman@redhat.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 2340b6d..c6d17ae 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))