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688903eb3e
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101 lines
4.2 KiB
C
101 lines
4.2 KiB
C
/* Copyright (C) 2003-2018 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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Contributed by Martin Schwidefsky <schwidefsky@de.ibm.com>, 2003.
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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 <endian.h>
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#include <errno.h>
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#include <sysdep.h>
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#include <futex-internal.h>
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#include <pthread.h>
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#include <pthreadP.h>
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#include <atomic.h>
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#include <stdint.h>
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#include <shlib-compat.h>
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#include <stap-probe.h>
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#include "pthread_cond_common.c"
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/* See __pthread_cond_wait for a high-level description of the algorithm. */
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int
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__pthread_cond_signal (pthread_cond_t *cond)
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{
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LIBC_PROBE (cond_signal, 1, cond);
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/* First check whether there are waiters. Relaxed MO is fine for that for
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the same reasons that relaxed MO is fine when observing __wseq (see
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below). */
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unsigned int wrefs = atomic_load_relaxed (&cond->__data.__wrefs);
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if (wrefs >> 3 == 0)
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return 0;
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int private = __condvar_get_private (wrefs);
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__condvar_acquire_lock (cond, private);
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/* Load the waiter sequence number, which represents our relative ordering
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to any waiters. Relaxed MO is sufficient for that because:
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1) We can pick any position that is allowed by external happens-before
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constraints. In particular, if another __pthread_cond_wait call
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happened before us, this waiter must be eligible for being woken by
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us. The only way do establish such a happens-before is by signaling
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while having acquired the mutex associated with the condvar and
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ensuring that the signal's critical section happens after the waiter.
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Thus, the mutex ensures that we see that waiter's __wseq increase.
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2) Once we pick a position, we do not need to communicate this to the
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program via a happens-before that we set up: First, any wake-up could
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be a spurious wake-up, so the program must not interpret a wake-up as
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an indication that the waiter happened before a particular signal;
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second, a program cannot detect whether a waiter has not yet been
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woken (i.e., it cannot distinguish between a non-woken waiter and one
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that has been woken but hasn't resumed execution yet), and thus it
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cannot try to deduce that a signal happened before a particular
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waiter. */
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unsigned long long int wseq = __condvar_load_wseq_relaxed (cond);
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unsigned int g1 = (wseq & 1) ^ 1;
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wseq >>= 1;
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bool do_futex_wake = false;
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/* If G1 is still receiving signals, we put the signal there. If not, we
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check if G2 has waiters, and if so, quiesce and switch G1 to the former
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G2; if this results in a new G1 with waiters (G2 might have cancellations
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already, see __condvar_quiesce_and_switch_g1), we put the signal in the
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new G1. */
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if ((cond->__data.__g_size[g1] != 0)
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|| __condvar_quiesce_and_switch_g1 (cond, wseq, &g1, private))
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{
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/* Add a signal. Relaxed MO is fine because signaling does not need to
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establish a happens-before relation (see above). We do not mask the
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release-MO store when initializing a group in
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__condvar_quiesce_and_switch_g1 because we use an atomic
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read-modify-write and thus extend that store's release sequence. */
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atomic_fetch_add_relaxed (cond->__data.__g_signals + g1, 2);
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cond->__data.__g_size[g1]--;
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/* TODO Only set it if there are indeed futex waiters. */
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do_futex_wake = true;
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}
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__condvar_release_lock (cond, private);
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if (do_futex_wake)
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futex_wake (cond->__data.__g_signals + g1, 1, private);
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return 0;
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}
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versioned_symbol (libpthread, __pthread_cond_signal, pthread_cond_signal,
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GLIBC_2_3_2);
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