Move assignment out of the CAS condition

Update

commit 49302b8fdf9103b6fc0a398678668a22fa19574c
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Thu Nov 11 06:54:01 2021 -0800

    Avoid extra load with CAS in __pthread_mutex_clocklock_common [BZ #28537]

    Replace boolean CAS with value CAS to avoid the extra load.

and

commit 0b82747dc48d5bf0871bdc6da8cb6eec1256355f
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Thu Nov 11 06:31:51 2021 -0800

    Avoid extra load with CAS in __pthread_mutex_lock_full [BZ #28537]

    Replace boolean CAS with value CAS to avoid the extra load.

by moving assignment out of the CAS condition.

(cherry picked from commit 120ac6d238825452e8024e2f627da33b2508dfd3)

diff --git a/nptl/pthread_mutex_lock.c b/nptl/pthread_mutex_lock.c
index ac8a472c08d6d0b84726204c47d027fece73eaff..2895926953eb7649bc2b032833a93472bd291c40 100644 (file)
--- a/nptl/pthread_mutex_lock.c
+++ b/nptl/pthread_mutex_lock.c
@@ -296,10 +296,9 @@ __pthread_mutex_lock_full (pthread_mutex_t *mutex)
             meantime.  */
          if ((oldval & FUTEX_WAITERS) == 0)
            {
-             int val;
-             if ((val = atomic_compare_and_exchange_val_acq
-                  (&mutex->__data.__lock, oldval | FUTEX_WAITERS,
-                   oldval)) != oldval)
+             int val = atomic_compare_and_exchange_val_acq
+               (&mutex->__data.__lock, oldval | FUTEX_WAITERS, oldval);
+             if (val != oldval)
                {
                  oldval = val;
                  continue;

diff --git a/nptl/pthread_mutex_timedlock.c b/nptl/pthread_mutex_timedlock.c
index 541ec189384ff1ca297e0fe24dd65cc0a63ef640..22174f74c9b314a6be615794c52653213d0d547b 100644 (file)
--- a/nptl/pthread_mutex_timedlock.c
+++ b/nptl/pthread_mutex_timedlock.c
@@ -247,10 +247,9 @@ __pthread_mutex_clocklock_common (pthread_mutex_t *mutex,
             meantime.  */
          if ((oldval & FUTEX_WAITERS) == 0)
            {
-             int val;
-             if ((val = atomic_compare_and_exchange_val_acq
-                  (&mutex->__data.__lock, oldval | FUTEX_WAITERS,
-                   oldval)) != oldval)
+             int val = atomic_compare_and_exchange_val_acq
+               (&mutex->__data.__lock, oldval | FUTEX_WAITERS, oldval);
+             if (val != oldval)
                {
                  oldval = val;
                  continue;