linux-kernel - [PATCH 1/2] locking/rwsem: No need to check for handoff bit if wait queue empty

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Date:   Fri, 18 Mar 2022 12:16:08 -0400
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>
Cc:     linux-kernel@...r.kernel.org, Waiman Long <longman@...hat.com>
Subject: [PATCH 1/2] 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>
---
 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 69aba4abe104..f71a9693d05a 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -977,12 +977,11 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int stat
 	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);
-- 
2.27.0