30891f35fa
We stopped adding "Contributed by" or similar lines in sources in 2012 in favour of git logs and keeping the Contributors section of the glibc manual up to date. Removing these lines makes the license header a bit more consistent across files and also removes the possibility of error in attribution when license blocks or files are copied across since the contributed-by lines don't actually reflect reality in those cases. Move all "Contributed by" and similar lines (Written by, Test by, etc.) into a new file CONTRIBUTED-BY to retain record of these contributions. These contributors are also mentioned in manual/contrib.texi, so we just maintain this additional record as a courtesy to the earlier developers. The following scripts were used to filter a list of files to edit in place and to clean up the CONTRIBUTED-BY file respectively. These were not added to the glibc sources because they're not expected to be of any use in future given that this is a one time task: https://gist.github.com/siddhesh/b5ecac94eabfd72ed2916d6d8157e7dc https://gist.github.com/siddhesh/15ea1f5e435ace9774f485030695ee02 Reviewed-by: Carlos O'Donell <carlos@redhat.com>
103 lines
4.3 KiB
C
103 lines
4.3 KiB
C
/* Copyright (C) 2003-2021 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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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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<https://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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libc_hidden_ver (___pthread_cond_signal, __pthread_cond_signal)
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#ifndef SHARED
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strong_alias (___pthread_cond_signal, __pthread_cond_signal)
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#endif
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