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/* Copyright (C) 2003-2020 Free Software Foundation, Inc.
This file is part of the GNU C Library.
Contributed by Martin Schwidefsky <schwidefsky@de.ibm.com>, 2003.
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
<https://www.gnu.org/licenses/>. */
#include <endian.h>
#include <errno.h>
#include <sysdep.h>
#include <futex-internal.h>
#include <pthread.h>
#include <pthreadP.h>
#include <atomic.h>
#include <stdint.h>
#include <shlib-compat.h>
#include <stap-probe.h>
#include "pthread_cond_common.c"
/* See __pthread_cond_wait for a high-level description of the algorithm. */
int
__pthread_cond_signal (pthread_cond_t *cond)
{
LIBC_PROBE (cond_signal, 1, cond);
/* First check whether there are waiters. Relaxed MO is fine for that for
the same reasons that relaxed MO is fine when observing __wseq (see
below). */
unsigned int wrefs = atomic_load_relaxed (&cond->__data.__wrefs);
if (wrefs >> 3 == 0)
return 0;
int private = __condvar_get_private (wrefs);
__condvar_acquire_lock (cond, private);
/* Load the waiter sequence number, which represents our relative ordering
to any waiters. Relaxed MO is sufficient for that because:
1) We can pick any position that is allowed by external happens-before
constraints. In particular, if another __pthread_cond_wait call
happened before us, this waiter must be eligible for being woken by
us. The only way do establish such a happens-before is by signaling
while having acquired the mutex associated with the condvar and
ensuring that the signal's critical section happens after the waiter.
Thus, the mutex ensures that we see that waiter's __wseq increase.
2) Once we pick a position, we do not need to communicate this to the
program via a happens-before that we set up: First, any wake-up could
be a spurious wake-up, so the program must not interpret a wake-up as
an indication that the waiter happened before a particular signal;
second, a program cannot detect whether a waiter has not yet been
woken (i.e., it cannot distinguish between a non-woken waiter and one
that has been woken but hasn't resumed execution yet), and thus it
cannot try to deduce that a signal happened before a particular
waiter. */
unsigned long long int wseq = __condvar_load_wseq_relaxed (cond);
unsigned int g1 = (wseq & 1) ^ 1;
wseq >>= 1;
bool do_futex_wake = false;
/* If G1 is still receiving signals, we put the signal there. If not, we
check if G2 has waiters, and if so, quiesce and switch G1 to the former
G2; if this results in a new G1 with waiters (G2 might have cancellations
already, see __condvar_quiesce_and_switch_g1), we put the signal in the
new G1. */
if ((cond->__data.__g_size[g1] != 0)
|| __condvar_quiesce_and_switch_g1 (cond, wseq, &g1, private))
{
/* Add a signal. Relaxed MO is fine because signaling does not need to
establish a happens-before relation (see above). We do not mask the
release-MO store when initializing a group in
__condvar_quiesce_and_switch_g1 because we use an atomic
read-modify-write and thus extend that store's release sequence. */
atomic_fetch_add_relaxed (cond->__data.__g_signals + g1, 2);
cond->__data.__g_size[g1]--;
/* TODO Only set it if there are indeed futex waiters. */
do_futex_wake = true;
}
__condvar_release_lock (cond, private);
if (do_futex_wake)
futex_wake (cond->__data.__g_signals + g1, 1, private);
return 0;
}
versioned_symbol (libpthread, __pthread_cond_signal, pthread_cond_signal,
GLIBC_2_3_2);
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