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The LSB of g_signals was unused. The LSB of g1_start was used to indicate which group is G2. This was used to always go to sleep in pthread_cond_wait if a waiter is in G2. A comment earlier in the file says that this is not correct to do: "Waiters cannot determine whether they are currently in G2 or G1 -- but they do not have to because all they are interested in is whether there are available signals" I either would have had to update the comment, or get rid of the check. I chose to get rid of the check. In fact I don't quite know why it was there. There will never be available signals for group G2, so we didn't need the special case. Even if there were, this would just be a spurious wake. This might have caught some cases where the count has wrapped around, but it wouldn't reliably do that, (and even if it did, why would you want to force a sleep in that case?) and we don't support that many concurrent waiters anyway. Getting rid of it allows us to use one more bit, making us more robust to wraparound. Signed-off-by: Malte Skarupke <malteskarupke@fastmail.fm> Reviewed-by: Carlos O'Donell <carlos@redhat.com>
101 lines
4.2 KiB
C
101 lines
4.2 KiB
C
/* Copyright (C) 2003-2025 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, switch G1 to the former G2; if this
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results in a new G1 with waiters (G2 might have cancellations already,
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see __condvar_switch_g1), we put the signal in the new G1. */
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if ((cond->__data.__g_size[g1] != 0)
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|| __condvar_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 __condvar_switch_g1
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because we use an atomic read-modify-write and thus extend that
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store's release sequence. */
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atomic_fetch_add_relaxed (cond->__data.__g_signals + g1, 1);
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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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