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The previous barrier implementation did not fulfill the POSIX requirements for when a barrier can be destroyed. Specifically, it was possible that threads that haven't noticed yet that their round is complete still access the barrier's memory, and that those accesses can happen after the barrier has been legally destroyed. The new algorithm does not have this issue, and it avoids using a lock internally.
62 lines
2.5 KiB
C
62 lines
2.5 KiB
C
/* Copyright (C) 2002-2016 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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Contributed by Ulrich Drepper <drepper@redhat.com>, 2002.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<http://www.gnu.org/licenses/>. */
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#include <errno.h>
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#include "pthreadP.h"
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#include <atomic.h>
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#include <futex-internal.h>
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int
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pthread_barrier_destroy (pthread_barrier_t *barrier)
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{
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struct pthread_barrier *bar = (struct pthread_barrier *) barrier;
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/* Destroying a barrier is only allowed if no thread is blocked on it.
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Thus, there is no unfinished round, and all modifications to IN will
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have happened before us (either because the calling thread took part
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in the most recent round and thus synchronized-with all other threads
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entering, or the program ensured this through other synchronization).
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We must wait until all threads that entered so far have confirmed that
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they have exited as well. To get the notification, pretend that we have
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reached the reset threshold. */
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unsigned int count = bar->count;
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unsigned int max_in_before_reset = BARRIER_IN_THRESHOLD
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- BARRIER_IN_THRESHOLD % count;
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/* Relaxed MO sufficient because the program must have ensured that all
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modifications happen-before this load (see above). */
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unsigned int in = atomic_load_relaxed (&bar->in);
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/* Trigger reset. The required acquire MO is below. */
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if (atomic_fetch_add_relaxed (&bar->out, max_in_before_reset - in) < in)
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{
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/* Not all threads confirmed yet that they have exited, so another
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thread will perform a reset. Wait until that has happened. */
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while (in != 0)
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{
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futex_wait_simple (&bar->in, in, bar->shared);
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in = atomic_load_relaxed (&bar->in);
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}
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}
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/* We must ensure that memory reuse happens after all prior use of the
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barrier (specifically, synchronize-with the reset of the barrier or the
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confirmation of threads leaving the barrier). */
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atomic_thread_fence_acquire ();
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return 0;
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}
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