linux-kernel - [tip: locking/core] locking/osq_lock: Use atomic_try_cmpxchg_release() in osq

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Message-ID: <172992812827.1442.5074326793772242513.tip-bot2@tip-bot2>
Date: Sat, 26 Oct 2024 07:35:28 -0000
From: "tip-bot2 for Uros Bizjak" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Uros Bizjak <ubizjak@...il.com>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>,
 Waiman Long <longman@...hat.com>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: locking/core] locking/osq_lock: Use
 atomic_try_cmpxchg_release() in osq_unlock()

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

Commit-ID:     0d75e0c420e52b4057a2de274054a5274209a2ae
Gitweb:        https://git.kernel.org/tip/0d75e0c420e52b4057a2de274054a5274209a2ae
Author:        Uros Bizjak <ubizjak@...il.com>
AuthorDate:    Tue, 01 Oct 2024 13:45:57 +02:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Fri, 25 Oct 2024 10:01:50 +02:00

locking/osq_lock: Use atomic_try_cmpxchg_release() in osq_unlock()

Replace this pattern in osq_unlock():

    atomic_cmpxchg(*ptr, old, new) == old

... with the simpler and faster:

    atomic_try_cmpxchg(*ptr, &old, new)

The x86 CMPXCHG instruction returns success in the ZF flag,
so this change saves a compare after the CMPXCHG.  The code
in the fast path of osq_unlock() improves from:

 11b:	31 c9                	xor    %ecx,%ecx
 11d:	8d 50 01             	lea    0x1(%rax),%edx
 120:	89 d0                	mov    %edx,%eax
 122:	f0 0f b1 0f          	lock cmpxchg %ecx,(%rdi)
 126:	39 c2                	cmp    %eax,%edx
 128:	75 05                	jne    12f <...>

to:

 12b:	31 d2                	xor    %edx,%edx
 12d:	83 c0 01             	add    $0x1,%eax
 130:	f0 0f b1 17          	lock cmpxchg %edx,(%rdi)
 134:	75 05                	jne    13b <...>

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Acked-by: Waiman Long <longman@...hat.com>
Link: https://lore.kernel.org/r/20241001114606.820277-1-ubizjak@gmail.com
---
 kernel/locking/osq_lock.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 75a6f61..b4233dc 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -215,8 +215,7 @@ void osq_unlock(struct optimistic_spin_queue *lock)
 	/*
 	 * Fast path for the uncontended case.
 	 */
-	if (likely(atomic_cmpxchg_release(&lock->tail, curr,
-					  OSQ_UNLOCKED_VAL) == curr))
+	if (atomic_try_cmpxchg_release(&lock->tail, &curr, OSQ_UNLOCKED_VAL))
 		return;
 
 	/*