mirror of
https://sourceware.org/git/glibc.git
synced 2024-11-26 06:50:07 +00:00
70 lines
2.5 KiB
C
70 lines
2.5 KiB
C
|
/* pthread_spin_lock -- lock a spin lock. Generic version.
|
||
|
|
Copyright (C) 2012 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 <atomic.h>
|
||
|
|
#include "pthreadP.h"
|
||
|
|
|
||
|
|
/* A machine-specific version can define SPIN_LOCK_READS_BETWEEN_CMPXCHG
|
||
|
|
to the number of plain reads that it's optimal to spin on between uses
|
||
|
|
of atomic_compare_and_exchange_val_acq. If spinning forever is optimal
|
||
|
|
then use -1. If no plain reads here would ever be optimal, use 0. */
|
||
|
|
#ifndef SPIN_LOCK_READS_BETWEEN_CMPXCHG
|
||
|
|
# warning machine-dependent file should define SPIN_LOCK_READS_BETWEEN_CMPXCHG
|
||
|
|
# define SPIN_LOCK_READS_BETWEEN_CMPXCHG 1000
|
||
|
|
#endif
|
||
|
|
|
||
|
|
int
|
||
|
|
pthread_spin_lock (pthread_spinlock_t *lock)
|
||
|
|
{
|
||
|
|
/* atomic_exchange usually takes less instructions than
|
||
|
|
atomic_compare_and_exchange. On the other hand,
|
||
|
|
atomic_compare_and_exchange potentially generates less bus traffic
|
||
|
|
when the lock is locked.
|
||
|
|
We assume that the first try mostly will be successful, and we use
|
||
|
|
atomic_exchange. For the subsequent tries we use
|
||
|
|
atomic_compare_and_exchange. */
|
||
|
|
if (atomic_exchange_acq (lock, 1) == 0)
|
||
|
|
return 0;
|
||
|
|
|
||
|
|
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.
|
||
|
|
On the other hand, we do want to update memory state on the local core
|
||
|
|
once in a while to avoid spinning indefinitely until some event that
|
||
|
|
will happen to update local memory as a side-effect. */
|
||
|
|
if (SPIN_LOCK_READS_BETWEEN_CMPXCHG >= 0)
|
||
|
|
{
|
||
|
|
int wait = SPIN_LOCK_READS_BETWEEN_CMPXCHG;
|
||
|
|
|
||
|
|
while (*lock != 0 && wait > 0)
|
||
|
|
--wait;
|
||
|
|
}
|
||
|
|
else
|
||
|
|
{
|
||
|
|
while (*lock != 0)
|
||
|
|
;
|
||
|
|
}
|
||
|
|
}
|
||
|
|
while (atomic_compare_and_exchange_val_acq (lock, 1, 0) != 0);
|
||
|
|
|
||
|
|
return 0;
|
||
|
|
}
|