mirror of
https://sourceware.org/git/glibc.git
synced 2024-11-14 01:00:07 +00:00
103 lines
4.3 KiB
C
103 lines
4.3 KiB
C
/* Copyright (C) 2003-2023 Free Software Foundation, Inc.
|
|
This file is part of the GNU C Library.
|
|
|
|
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);
|
|
libc_hidden_ver (___pthread_cond_signal, __pthread_cond_signal)
|
|
#ifndef SHARED
|
|
strong_alias (___pthread_cond_signal, __pthread_cond_signal)
|
|
#endif
|