[tip:locking/core] locking/qspinlock: Use atomic_sub_return_release() in queued_spin_unlock()

[tip-bot for Pan Xinhui <tipbot@zytor.com>]

Commit-ID:  ca50e426f96c905e7d14a9c7a6bd4e0330516047
Gitweb:     http://git.kernel.org/tip/ca50e426f96c905e7d14a9c7a6bd4e0330516047
Author:     Pan Xinhui <xinhui.pan@linux.vnet.ibm.com>
AuthorDate: Fri, 3 Jun 2016 16:38:14 +0800
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 8 Jun 2016 15:17:01 +0200

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@infradead.org>
Signed-off-by: Pan Xinhui <xinhui.pan@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: arnd@arndb.de
Cc: waiman.long@hp.com
Link: http://lkml.kernel.org/r/1464943094-3129-1-git-send-email-xinhui.pan@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 include/asm-generic/qspinlock.h | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qspinlock.h
index 05f05f1..9f0681b 100644
--- a/include/asm-generic/qspinlock.h
+++ b/include/asm-generic/qspinlock.h
@@ -111,10 +111,9 @@ 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
+	 * unlock() needs release semantics:
 	 */
-	smp_mb__before_atomic();
-	atomic_sub(_Q_LOCKED_VAL, &lock->val);
+	(void)atomic_sub_return_release(_Q_LOCKED_VAL, &lock->val);
 }
 #endif