linux-kernel - [tip: locking/core] locking/rwsem: No need to check for handoff bit if wait queue empty

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Message-ID: <164914778225.389.644461284127262702.tip-bot2@tip-bot2>
Date:   Tue, 05 Apr 2022 08:36:22 -0000
From:   "tip-bot2 for Waiman Long" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Waiman Long <longman@...hat.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: locking/core] locking/rwsem: No need to check for handoff bit
 if wait queue empty

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

Commit-ID:     f9e21aa9e6fb11355e54c8949a390d49ca21cde1
Gitweb:        https://git.kernel.org/tip/f9e21aa9e6fb11355e54c8949a390d49ca21cde1
Author:        Waiman Long <longman@...hat.com>
AuthorDate:    Tue, 22 Mar 2022 11:20:57 -04:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Tue, 05 Apr 2022 10:24:34 +02:00

locking/rwsem: No need to check for handoff bit if wait queue empty

Since commit d257cc8cb8d5 ("locking/rwsem: Make handoff bit handling
more consistent"), the handoff bit is always cleared if the wait queue
becomes empty. There is no need to check for RWSEM_FLAG_HANDOFF when
the wait list is known to be empty.

Signed-off-by: Waiman Long <longman@...hat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/20220322152059.2182333-2-longman@redhat.com
---
 kernel/locking/rwsem.c |  9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index acde5d6..b077b1b 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -977,12 +977,11 @@ queue:
 	if (list_empty(&sem->wait_list)) {
 		/*
 		 * In case the wait queue is empty and the lock isn't owned
-		 * by a writer or has the handoff bit set, this reader can
-		 * exit the slowpath and return immediately as its
-		 * RWSEM_READER_BIAS has already been set in the count.
+		 * by a writer, this reader can exit the slowpath and return
+		 * immediately as its RWSEM_READER_BIAS has already been set
+		 * in the count.
 		 */
-		if (!(atomic_long_read(&sem->count) &
-		     (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
+		if (!(atomic_long_read(&sem->count) & RWSEM_WRITER_MASK)) {
 			/* Provide lock ACQUIRE */
 			smp_acquire__after_ctrl_dep();
 			raw_spin_unlock_irq(&sem->wait_lock);