mirror of
https://sourceware.org/git/glibc.git
synced 2024-11-22 21:10:07 +00:00
81 lines
3.1 KiB
C
81 lines
3.1 KiB
C
/* pthread_spin_lock -- lock a spin lock. Generic version.
|
|
Copyright (C) 2012-2020 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;
|
|
}
|