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/* pthread_spin_lock -- lock a spin lock. Generic version.
Copyright (C) 2012-2021 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
<https://www.gnu.org/licenses/>. */
#include <atomic.h>
#include "pthreadP.h"
int
pthread_spin_lock (pthread_spinlock_t *lock)
{
int val = 0;
/* We assume that the first try mostly will be successful, thus we use
atomic_exchange if it is not implemented by a CAS loop (we also assume
that atomic_exchange can be faster if it succeeds, see
ATOMIC_EXCHANGE_USES_CAS). Otherwise, we use a weak CAS and not an
exchange so we bail out after the first failed attempt to change the
state. For the subsequent attempts we use atomic_compare_and_exchange
after we observe that the lock is not acquired.
See also comment in pthread_spin_trylock.
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))
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
{
/* The lock is contended and we need to wait. Going straight back
to cmpxchg is not a good idea on many targets as that will force
expensive memory synchronizations among processors and penalize other
running threads.
There is no technical reason for throwing in a CAS every now and then,
and so far we have no evidence that it can improve performance.
If that would be the case, we have to adjust other spin-waiting loops
elsewhere, too!
Thus we use relaxed MO reads until we observe the lock to not be
acquired anymore. */
do
{
/* TODO Back-off. */
atomic_spin_nop ();
val = atomic_load_relaxed (lock);
}
while (val != 0);
/* 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));
return 0;
}
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