mirror of
git://sourceware.org/git/glibc.git
synced 2024-11-21 01:12:26 +08:00
688903eb3e
* All files with FSF copyright notices: Update copyright dates using scripts/update-copyrights. * locale/programs/charmap-kw.h: Regenerated. * locale/programs/locfile-kw.h: Likewise.
80 lines
3.3 KiB
C
80 lines
3.3 KiB
C
/* pthread_spin_trylock -- trylock a spin lock. Generic version.
|
|
Copyright (C) 2012-2018 Free Software Foundation, Inc.
|
|
This file is part of the GNU C Library.
|
|
|
|
The GNU C Library is free software; you can redistribute it and/or
|
|
modify it under the terms of the GNU Lesser General Public
|
|
License as published by the Free Software Foundation; either
|
|
version 2.1 of the License, or (at your option) any later version.
|
|
|
|
The GNU C Library is distributed in the hope that it will be useful,
|
|
but WITHOUT ANY WARRANTY; without even the implied warranty of
|
|
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
|
|
Lesser General Public License for more details.
|
|
|
|
You should have received a copy of the GNU Lesser General Public
|
|
License along with the GNU C Library; if not, see
|
|
<http://www.gnu.org/licenses/>. */
|
|
|
|
#include <errno.h>
|
|
#include <atomic.h>
|
|
#include "pthreadP.h"
|
|
|
|
int
|
|
pthread_spin_trylock (pthread_spinlock_t *lock)
|
|
{
|
|
/* For the spin try lock, we have the following possibilities:
|
|
|
|
1) If we assume that trylock will most likely succeed in practice:
|
|
* We just do an exchange.
|
|
|
|
2) If we want to bias towards cases where trylock succeeds, but don't
|
|
rule out contention:
|
|
* If exchange is not implemented by a CAS loop, and exchange is faster
|
|
than CAS, do an exchange.
|
|
* If exchange is implemented by a CAS loop, use a weak CAS and not an
|
|
exchange so we bail out after the first failed attempt to change the state.
|
|
|
|
3) If we expect contention to be likely:
|
|
* If CAS always brings the cache line into an exclusive state even if the
|
|
spinlock is already acquired, then load the value first with
|
|
atomic_load_relaxed and test if lock is not acquired. Then do 2).
|
|
|
|
We assume that 2) is the common case, and that this won't be slower than
|
|
1) in the common case.
|
|
|
|
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 (atomic_exchange_acquire (lock, 1) == 0)
|
|
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. */
|
|
do
|
|
{
|
|
int val = 0;
|
|
if (atomic_compare_exchange_weak_acquire (lock, &val, 1))
|
|
return 0;
|
|
}
|
|
/* atomic_compare_exchange_weak_acquire can fail spuriously. Whereas
|
|
C++11 and C11 make it clear that trylock operations can fail spuriously,
|
|
POSIX does not explicitly specify this; it only specifies that failing
|
|
synchronization operations do not need to have synchronization effects
|
|
themselves, but a spurious failure is something that could contradict a
|
|
happens-before established earlier (e.g., that we need to observe that
|
|
the lock is acquired). Therefore, we emulate a strong CAS by simply
|
|
checking with a relaxed MO load that the lock is really acquired before
|
|
returning EBUSY; the additional overhead this may cause is on the slow
|
|
path. */
|
|
while (atomic_load_relaxed (lock) == 0);
|
|
#endif
|
|
|
|
return EBUSY;
|
|
}
|