85 lines
2.7 KiB
C
85 lines
2.7 KiB
C
/* Copyright (C) 2005-2020 Free Software Foundation, Inc.
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Contributed by Richard Henderson <rth@redhat.com>.
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This file is part of the GNU Offloading and Multi Processing Library
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(libgomp).
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Libgomp is free software; you can redistribute it and/or modify it
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under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3, or (at your option)
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any later version.
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Libgomp is distributed in the hope that it will be useful, but WITHOUT ANY
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WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
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FOR A PARTICULAR PURPOSE. See the GNU General Public License for
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more details.
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Under Section 7 of GPL version 3, you are granted additional
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permissions described in the GCC Runtime Library Exception, version
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3.1, as published by the Free Software Foundation.
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You should have received a copy of the GNU General Public License and
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a copy of the GCC Runtime Library Exception along with this program;
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see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
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<http://www.gnu.org/licenses/>. */
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/* This is a Linux specific implementation of a semaphore synchronization
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mechanism for libgomp. This type is private to the library. This
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implementation uses atomic instructions and the futex syscall. */
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#include "wait.h"
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void
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gomp_sem_wait_slow (gomp_sem_t *sem, int count)
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{
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/* First loop spins a while. */
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while (count == 0)
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if (do_spin (sem, 0)
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/* Spin timeout, nothing changed. Set waiting flag. */
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&& __atomic_compare_exchange_n (sem, &count, SEM_WAIT, false,
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MEMMODEL_ACQUIRE, MEMMODEL_RELAXED))
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{
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futex_wait (sem, SEM_WAIT);
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count = *sem;
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break;
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}
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/* Something changed. If it wasn't the wait flag, we're good to go. */
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else if (__builtin_expect (((count = *sem) & SEM_WAIT) == 0 && count != 0,
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1))
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{
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if (__atomic_compare_exchange_n (sem, &count, count - SEM_INC, false,
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MEMMODEL_ACQUIRE, MEMMODEL_RELAXED))
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return;
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}
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/* Second loop waits until semaphore is posted. We always exit this
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loop with wait flag set, so next post will awaken a thread. */
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while (1)
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{
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unsigned int wake = count & ~SEM_WAIT;
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int newval = SEM_WAIT;
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if (wake != 0)
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newval |= wake - SEM_INC;
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if (__atomic_compare_exchange_n (sem, &count, newval, false,
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MEMMODEL_ACQUIRE, MEMMODEL_RELAXED))
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{
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if (wake != 0)
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{
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/* If we can wake more threads, do so now. */
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if (wake > SEM_INC)
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gomp_sem_post_slow (sem);
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break;
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}
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do_wait (sem, SEM_WAIT);
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count = *sem;
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}
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}
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}
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void
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gomp_sem_post_slow (gomp_sem_t *sem)
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{
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futex_wake (sem, 1);
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}
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