linux-kernel - [PATCH] locking/qspinlock: Use atomic_sub_return_release in queued_spin

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Message-Id: <1464943094-3129-1-git-send-email-xinhui.pan@linux.vnet.ibm.com>
Date:	Fri,  3 Jun 2016 16:38:14 +0800
From:	Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Cc:	arnd@...db.de, peterz@...radead.org, waiman.long@...com,
	Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>
Subject: [PATCH] locking/qspinlock: Use atomic_sub_return_release in queued_spin_unlock

The existing version uses a heavy barrier while only release semantics
is required. So use atomic_sub_return_release instead.

Suggested-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>
---
 include/asm-generic/qspinlock.h | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qspinlock.h
index 35a52a8..8947cd2 100644
--- a/include/asm-generic/qspinlock.h
+++ b/include/asm-generic/qspinlock.h
@@ -92,10 +92,9 @@ static __always_inline void queued_spin_lock(struct qspinlock *lock)
 static __always_inline void queued_spin_unlock(struct qspinlock *lock)
 {
 	/*
-	 * smp_mb__before_atomic() in order to guarantee release semantics
-	 */
-	smp_mb__before_atomic();
-	atomic_sub(_Q_LOCKED_VAL, &lock->val);
+	* unlock() need release semantics
+	*/
+	(void)atomic_sub_return_release(_Q_LOCKED_VAL, &lock->val);
 }
 #endif
 
-- 
1.9.1