[PATCH v5 3/3] Optimize CAS in __pthread_mutex_lock_full [BZ #28537]

["H.J. Lu" <hjl.tools@gmail.com>]

1. Do an atomic load and skip CAS if compare may fail to reduce cache
line bouncing on contended locks.
2. Replace atomic_compare_and_exchange_bool_acq with
atomic_compare_and_exchange_val_acq to avoid the extra load.
---
 nptl/pthread_mutex_lock.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c
index d7e8efedd2..24ff1772cd 100644
--- a/nptl/pthread_mutex_lock.c
+++ b/nptl/pthread_mutex_lock.c
@@ -297,12 +297,13 @@ __pthread_mutex_lock_full (pthread_mutex_t *mutex)
 	     meantime.  */
 	  if ((oldval & FUTEX_WAITERS) == 0)
 	    {
-	      if (atomic_compare_and_exchange_bool_acq (&mutex->__data.__lock,
-							oldval | FUTEX_WAITERS,
-							oldval)
-		  != 0)
+	      int val = atomic_load_relaxed (&mutex->__data.__lock);
+	      if (val != oldval
+		  || ((val = atomic_compare_and_exchange_val_acq
+			 (&mutex->__data.__lock, oldval | FUTEX_WAITERS,
+			  oldval)) != oldval))
 		{
-		  oldval = mutex->__data.__lock;
+		  oldval = val;
 		  continue;
 		}
 	      oldval |= FUTEX_WAITERS;
-- 
2.33.1