[PATCH] mutex: while spinning read count before attempting cmpxchg

[Andi Kleen]

Under heavy contention it's better to read first before trying
to do an atomic operation on the interconnect.

This gives a few percent improvement for the mutex CPU time
under heavy contention and likely saves some power too.

Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>

diff --git a/kernel/mutex.c b/kernel/mutex.c
index d607ed5..1abffa9 100644
--- a/kernel/mutex.c
+++ b/kernel/mutex.c
@@ -170,7 +170,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 		if (owner && !mutex_spin_on_owner(lock, owner))
 			break;
 
-		if (atomic_cmpxchg(&lock->count, 1, 0) == 1) {
+		if (atomic_read(&lock->count) == 1 && 
+		    atomic_cmpxchg(&lock->count, 1, 0) == 1) {
 			lock_acquired(&lock->dep_map, ip);
 			mutex_set_owner(lock);
 			preempt_enable();



-- 
ak@xxxxxxxxxxxxxxx -- Speaking for myself only
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/