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80 lines
3.3 KiB
C
80 lines
3.3 KiB
C
/* pthread_spin_trylock -- trylock a spin lock. Generic version.
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Copyright (C) 2012-2018 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<http://www.gnu.org/licenses/>. */
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#include <errno.h>
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#include <atomic.h>
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#include "pthreadP.h"
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int
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pthread_spin_trylock (pthread_spinlock_t *lock)
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{
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/* For the spin try lock, we have the following possibilities:
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1) If we assume that trylock will most likely succeed in practice:
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* We just do an exchange.
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2) If we want to bias towards cases where trylock succeeds, but don't
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rule out contention:
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* If exchange is not implemented by a CAS loop, and exchange is faster
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than CAS, do an exchange.
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* If exchange is implemented by a CAS loop, use a weak CAS and not an
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exchange so we bail out after the first failed attempt to change the state.
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3) If we expect contention to be likely:
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* If CAS always brings the cache line into an exclusive state even if the
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spinlock is already acquired, then load the value first with
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atomic_load_relaxed and test if lock is not acquired. Then do 2).
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We assume that 2) is the common case, and that this won't be slower than
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1) in the common case.
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We use acquire MO to synchronize-with the release MO store in
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pthread_spin_unlock, and thus ensure that prior critical sections
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happen-before this critical section. */
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#if ! ATOMIC_EXCHANGE_USES_CAS
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/* Try to acquire the lock with an exchange instruction as this architecture
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has such an instruction and we assume it is faster than a CAS.
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The acquisition succeeds if the lock is not in an acquired state. */
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if (atomic_exchange_acquire (lock, 1) == 0)
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return 0;
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#else
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/* Try to acquire the lock with a CAS instruction as this architecture
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has no exchange instruction. The acquisition succeeds if the lock is not
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acquired. */
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do
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{
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int val = 0;
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if (atomic_compare_exchange_weak_acquire (lock, &val, 1))
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return 0;
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}
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/* atomic_compare_exchange_weak_acquire can fail spuriously. Whereas
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C++11 and C11 make it clear that trylock operations can fail spuriously,
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POSIX does not explicitly specify this; it only specifies that failing
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synchronization operations do not need to have synchronization effects
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themselves, but a spurious failure is something that could contradict a
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happens-before established earlier (e.g., that we need to observe that
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the lock is acquired). Therefore, we emulate a strong CAS by simply
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checking with a relaxed MO load that the lock is really acquired before
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returning EBUSY; the additional overhead this may cause is on the slow
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path. */
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while (atomic_load_relaxed (lock) == 0);
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#endif
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return EBUSY;
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}
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