[v1] nptl: Continue use arch prefered atomic exchange in spinlock loop
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Description |
| dj/TryBot-32bit |
success
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Build for i686
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| dj/TryBot-apply_patch |
success
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Patch applied to master at the time it was sent
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Commit Message
Despite using the preferred atomic exchange in the initial check, the
loop was unconditionally using CAS which is not desired on some
architectures (those that didn't set `ATOMIC_EXCHANGE_USES_CAS`).
Tested in x86-64 with `ATOMIC_EXCHANGE_USE_CAS` as `0` and `1`.
---
nptl/pthread_spin_lock.c | 29 ++++++++++++++++-------------
1 file changed, 16 insertions(+), 13 deletions(-)
@@ -20,6 +20,20 @@
#include "pthreadP.h"
#include <shlib-compat.h>
+#if ATOMIC_EXCHANGE_USES_CAS
+/* Try to acquire the lock with a CAS instruction as this architecture
+ has no exchange instruction. The acquisition succeeds if the lock is not
+ acquired. */
+# define pthread_spin_lock_grab_lock(mem, val, c) \
+ atomic_compare_exchange_weak_acquire (lock, val, 1))
+#else
+/* Try to acquire the lock with an exchange instruction as this architecture
+ has such an instruction and we assume it is faster than a CAS.
+ The acquisition succeeds if the lock is not in an acquired state. */
+# define pthread_spin_lock_grab_lock(mem, val, c) \
+ (atomic_exchange_acquire (lock, 1) == 0)
+#endif
+
int
__pthread_spin_lock (pthread_spinlock_t *lock)
{
@@ -36,19 +50,8 @@ __pthread_spin_lock (pthread_spinlock_t *lock)
We use acquire MO to synchronize-with the release MO store in
pthread_spin_unlock, and thus ensure that prior critical sections
happen-before this critical section. */
-#if ! ATOMIC_EXCHANGE_USES_CAS
- /* Try to acquire the lock with an exchange instruction as this architecture
- has such an instruction and we assume it is faster than a CAS.
- The acquisition succeeds if the lock is not in an acquired state. */
- if (__glibc_likely (atomic_exchange_acquire (lock, 1) == 0))
+ if (__glibc_likely (pthread_spin_lock_grab_lock (lock, &val, 1)))
return 0;
-#else
- /* Try to acquire the lock with a CAS instruction as this architecture
- has no exchange instruction. The acquisition succeeds if the lock is not
- acquired. */
- if (__glibc_likely (atomic_compare_exchange_weak_acquire (lock, &val, 1)))
- return 0;
-#endif
do
{
@@ -75,7 +78,7 @@ __pthread_spin_lock (pthread_spinlock_t *lock)
/* We need acquire memory order here for the same reason as mentioned
for the first try to lock the spinlock. */
}
- while (!atomic_compare_exchange_weak_acquire (lock, &val, 1));
+ while (!pthread_spin_lock_grab_lock (lock, &val, 1));
return 0;
}