[v5,2/3] Reduce CAS in __pthread_mutex_lock_full [BZ #28537]
Checks
Context |
Check |
Description |
dj/TryBot-apply_patch |
success
|
Patch applied to master at the time it was sent
|
Commit Message
Change __pthread_mutex_lock_full to do an atomic load and skip CAS if
compare may fail to reduce cache line bouncing on contended locks.
---
nptl/pthread_mutex_lock.c | 38 +++++++++++++++++++++++++-------------
1 file changed, 25 insertions(+), 13 deletions(-)
@@ -223,13 +223,13 @@ __pthread_mutex_lock_full (pthread_mutex_t *mutex)
newval |= (oldval & FUTEX_WAITERS) | assume_other_futex_waiters;
#endif
- newval
- = atomic_compare_and_exchange_val_acq (&mutex->__data.__lock,
- newval, oldval);
-
- if (newval != oldval)
+ int val = atomic_load_relaxed (&mutex->__data.__lock);
+ if (val != oldval
+ || ((val = atomic_compare_and_exchange_val_acq
+ (&mutex->__data.__lock, newval, oldval))
+ != oldval))
{
- oldval = newval;
+ oldval = val;
continue;
}
@@ -411,11 +411,15 @@ __pthread_mutex_lock_full (pthread_mutex_t *mutex)
# ifdef NO_INCR
newval |= FUTEX_WAITERS;
# endif
+ oldval = atomic_load_relaxed (&mutex->__data.__lock);
+ if (oldval != 0)
+ goto locked_mutex;
oldval = atomic_compare_and_exchange_val_acq (&mutex->__data.__lock,
newval, 0);
if (oldval != 0)
{
+ locked_mutex:;
/* The mutex is locked. The kernel will now take care of
everything. */
int private = (robust
@@ -554,6 +558,10 @@ __pthread_mutex_lock_full (pthread_mutex_t *mutex)
ceilval = ceiling << PTHREAD_MUTEX_PRIO_CEILING_SHIFT;
oldprio = ceiling;
+ oldval = atomic_load_relaxed (&mutex->__data.__lock);
+ if (oldval != ceilval)
+ goto ceilval_failed;
+
oldval
= atomic_compare_and_exchange_val_acq (&mutex->__data.__lock,
#ifdef NO_INCR
@@ -568,10 +576,13 @@ __pthread_mutex_lock_full (pthread_mutex_t *mutex)
do
{
- oldval
- = atomic_compare_and_exchange_val_acq (&mutex->__data.__lock,
- ceilval | 2,
- ceilval | 1);
+ oldval = atomic_load_relaxed (&mutex->__data.__lock);
+ ceilval_failed:
+ if (oldval == (ceilval | 1))
+ oldval
+ = atomic_compare_and_exchange_val_acq (&mutex->__data.__lock,
+ ceilval | 2,
+ ceilval | 1);
if ((oldval & PTHREAD_MUTEX_PRIO_CEILING_MASK) != ceilval)
break;
@@ -581,9 +592,10 @@ __pthread_mutex_lock_full (pthread_mutex_t *mutex)
ceilval | 2,
PTHREAD_MUTEX_PSHARED (mutex));
}
- while (atomic_compare_and_exchange_val_acq (&mutex->__data.__lock,
- ceilval | 2, ceilval)
- != ceilval);
+ while (atomic_load_relaxed (&mutex->__data.__lock) != ceilval
+ || (atomic_compare_and_exchange_val_acq (&mutex->__data.__lock,
+ ceilval | 2, ceilval)
+ != ceilval));
}
while ((oldval & PTHREAD_MUTEX_PRIO_CEILING_MASK) != ceilval);