linux-kernel - [tip:locking/core] locking/osq: Relax atomic semantics

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Date:	Fri, 18 Sep 2015 01:50:35 -0700
From:	tip-bot for Davidlohr Bueso <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Waiman.Long@....com, hpa@...or.com, dave@...olabs.net,
	mingo@...nel.org, akpm@...ux-foundation.org, dbueso@...e.de,
	paulmck@...ux.vnet.ibm.com, tglx@...utronix.de,
	peterz@...radead.org
Subject: [tip:locking/core] locking/osq: Relax atomic semantics

Commit-ID:  c55a6ffa6285e29f874ed403979472631ec70bff
Gitweb:     http://git.kernel.org/tip/c55a6ffa6285e29f874ed403979472631ec70bff
Author:     Davidlohr Bueso <dave@...olabs.net>
AuthorDate: Mon, 14 Sep 2015 00:37:24 -0700
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Fri, 18 Sep 2015 09:27:29 +0200

locking/osq: Relax atomic semantics

... by using acquire/release for ops around the lock->tail. As such,
weakly ordered archs can benefit from more relaxed use of barriers
when issuing atomics.

Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Waiman Long <Waiman.Long@....com>
Link: http://lkml.kernel.org/r/1442216244-4409-3-git-send-email-dave@stgolabs.net
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/locking/osq_lock.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index dc85ee2..d092a0c 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -50,7 +50,7 @@ osq_wait_next(struct optimistic_spin_queue *lock,
 
 	for (;;) {
 		if (atomic_read(&lock->tail) == curr &&
-		    atomic_cmpxchg(&lock->tail, curr, old) == curr) {
+		    atomic_cmpxchg_acquire(&lock->tail, curr, old) == curr) {
 			/*
 			 * We were the last queued, we moved @lock back. @prev
 			 * will now observe @lock and will complete its
@@ -92,7 +92,11 @@ bool osq_lock(struct optimistic_spin_queue *lock)
 	node->next = NULL;
 	node->cpu = curr;
 
-	old = atomic_xchg(&lock->tail, curr);
+	/*
+	 * ACQUIRE semantics, pairs with corresponding RELEASE
+	 * in unlock() uncontended, or fastpath.
+	 */
+	old = atomic_xchg_acquire(&lock->tail, curr);
 	if (old == OSQ_UNLOCKED_VAL)
 		return true;
 
@@ -184,7 +188,8 @@ void osq_unlock(struct optimistic_spin_queue *lock)
 	/*
 	 * Fast path for the uncontended case.
 	 */
-	if (likely(atomic_cmpxchg(&lock->tail, curr, OSQ_UNLOCKED_VAL) == curr))
+	if (likely(atomic_cmpxchg_release(&lock->tail, curr,
+					  OSQ_UNLOCKED_VAL) == curr))
 		return;
 
 	/*
--
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