From b3e0b1b6d841a4b2f64fc09ea728913da8218424 Mon Sep 17 00:00:00 2001
From: Peter Zijlstra <peterz@infradead.org>
Date: Fri, 16 Oct 2015 14:39:38 +0200
Subject: locking, sched: Introduce smp_cond_acquire() and use it

Introduce smp_cond_acquire() which combines a control dependency and a
read barrier to form acquire semantics.

This primitive has two benefits:

 - it documents control dependencies,
 - its typically cheaper than using smp_load_acquire() in a loop.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/locking/qspinlock.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

(limited to 'kernel/locking')

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 986207887def..ed9d96708f93 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -433,8 +433,7 @@ queue:
 	 *
 	 */
 	pv_wait_head(lock, node);
-	while ((val = smp_load_acquire(&lock->val.counter)) & _Q_LOCKED_PENDING_MASK)
-		cpu_relax();
+	smp_cond_acquire(!((val = atomic_read(&lock->val)) & _Q_LOCKED_PENDING_MASK));
 
 	/*
 	 * claim the lock:
-- 
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