linux-kernel - [PATCH] locking/osq_lock: Use try_cmpxchg() family of functions instead of cmpxchg()

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Message-ID: <20240719112130.59260-1-ubizjak@gmail.com>
Date: Fri, 19 Jul 2024 13:21:08 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: linux-kernel@...r.kernel.org
Cc: Uros Bizjak <ubizjak@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Will Deacon <will@...nel.org>,
	Waiman Long <longman@...hat.com>,
	Boqun Feng <boqun.feng@...il.com>
Subject: [PATCH] locking/osq_lock: Use try_cmpxchg() family of functions instead of cmpxchg()

Replace this pattern in osq_lock.c:

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

... with the simpler and faster:

    try_cmpxchg(*ptr, &old, new)

The x86 CMPXCHG instruction returns success in the ZF flag, so this change
saves a compare after the CMPXCHG.  As an example, the code in 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>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Will Deacon <will@...nel.org>
Cc: Waiman Long <longman@...hat.com>
Cc: Boqun Feng <boqun.feng@...il.com>
---
 kernel/locking/osq_lock.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 75a6f6133866..4f89ac8e6a69 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -58,8 +58,10 @@ osq_wait_next(struct optimistic_spin_queue *lock,
 	int curr = encode_cpu(smp_processor_id());
 
 	for (;;) {
-		if (atomic_read(&lock->tail) == curr &&
-		    atomic_cmpxchg_acquire(&lock->tail, curr, old_cpu) == curr) {
+		int tmp = curr;
+
+		if (atomic_read(&lock->tail) == tmp &&
+		    atomic_try_cmpxchg_acquire(&lock->tail, &tmp, old_cpu)) {
 			/*
 			 * We were the last queued, we moved @lock back. @prev
 			 * will now observe @lock and will complete its
@@ -157,12 +159,14 @@ bool osq_lock(struct optimistic_spin_queue *lock)
 	 */
 
 	for (;;) {
+		struct optimistic_spin_node *tmp = node;
+
 		/*
 		 * cpu_relax() below implies a compiler barrier which would
 		 * prevent this comparison being optimized away.
 		 */
-		if (data_race(prev->next) == node &&
-		    cmpxchg(&prev->next, node, NULL) == node)
+		if (data_race(prev->next) == tmp &&
+		    try_cmpxchg(&prev->next, &tmp, NULL))
 			break;
 
 		/*
@@ -215,8 +219,7 @@ 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;
 
 	/*
-- 
2.42.0