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/* Copyright (C) 2005-2014 Free Software Foundation, Inc.
Contributed by Richard Henderson <rth@redhat.com>.
This file is part of the GNU OpenMP Library (libgomp).
Libgomp is free software; you can redistribute it and/or modify it
under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3, or (at your option)
any later version.
Libgomp 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 General Public License for
more details.
Under Section 7 of GPL version 3, you are granted additional
permissions described in the GCC Runtime Library Exception, version
3.1, as published by the Free Software Foundation.
You should have received a copy of the GNU General Public License and
a copy of the GCC Runtime Library Exception along with this program;
see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
<http://www.gnu.org/licenses/>. */
/* This is a Linux specific implementation of a semaphore synchronization
mechanism for libgomp. This type is private to the library. This
implementation uses atomic instructions and the futex syscall. */
#include "wait.h"
void
gomp_sem_wait_slow (gomp_sem_t *sem, int count)
{
/* First loop spins a while. */
while (count == 0)
if (do_spin (sem, 0)
/* Spin timeout, nothing changed. Set waiting flag. */
&& __atomic_compare_exchange_n (sem, &count, SEM_WAIT, false,
MEMMODEL_ACQUIRE, MEMMODEL_RELAXED))
{
futex_wait (sem, SEM_WAIT);
count = *sem;
break;
}
/* Something changed. If it wasn't the wait flag, we're good to go. */
else if (__builtin_expect (((count = *sem) & SEM_WAIT) == 0 && count != 0,
1))
{
if (__atomic_compare_exchange_n (sem, &count, count - SEM_INC, false,
MEMMODEL_ACQUIRE, MEMMODEL_RELAXED))
return;
}
/* Second loop waits until semaphore is posted. We always exit this
loop with wait flag set, so next post will awaken a thread. */
while (1)
{
unsigned int wake = count & ~SEM_WAIT;
int newval = SEM_WAIT;
if (wake != 0)
newval |= wake - SEM_INC;
if (__atomic_compare_exchange_n (sem, &count, newval, false,
MEMMODEL_ACQUIRE, MEMMODEL_RELAXED))
{
if (wake != 0)
{
/* If we can wake more threads, do so now. */
if (wake > SEM_INC)
gomp_sem_post_slow (sem);
break;
}
do_wait (sem, SEM_WAIT);
count = *sem;
}
}
}
void
gomp_sem_post_slow (gomp_sem_t *sem)
{
futex_wake (sem, 1);
}
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