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81 lines
3.1 KiB
C
81 lines
3.1 KiB
C
/* pthread_spin_lock -- lock a spin lock. Generic version.
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Copyright (C) 2012-2019 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 <atomic.h>
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#include "pthreadP.h"
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int
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pthread_spin_lock (pthread_spinlock_t *lock)
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{
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int val = 0;
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/* We assume that the first try mostly will be successful, thus we use
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atomic_exchange if it is not implemented by a CAS loop (we also assume
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that atomic_exchange can be faster if it succeeds, see
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ATOMIC_EXCHANGE_USES_CAS). Otherwise, we use a weak CAS and not an
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exchange so we bail out after the first failed attempt to change the
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state. For the subsequent attempts we use atomic_compare_and_exchange
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after we observe that the lock is not acquired.
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See also comment in pthread_spin_trylock.
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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 (__glibc_likely (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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if (__glibc_likely (atomic_compare_exchange_weak_acquire (lock, &val, 1)))
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return 0;
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#endif
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do
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{
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/* The lock is contended and we need to wait. Going straight back
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to cmpxchg is not a good idea on many targets as that will force
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expensive memory synchronizations among processors and penalize other
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running threads.
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There is no technical reason for throwing in a CAS every now and then,
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and so far we have no evidence that it can improve performance.
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If that would be the case, we have to adjust other spin-waiting loops
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elsewhere, too!
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Thus we use relaxed MO reads until we observe the lock to not be
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acquired anymore. */
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do
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{
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/* TODO Back-off. */
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atomic_spin_nop ();
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val = atomic_load_relaxed (lock);
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}
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while (val != 0);
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/* We need acquire memory order here for the same reason as mentioned
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for the first try to lock the spinlock. */
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}
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while (!atomic_compare_exchange_weak_acquire (lock, &val, 1));
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return 0;
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}
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