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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>
94 lines
3.3 KiB
C
94 lines
3.3 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 <stap-probe.h>
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#include <atomic.h>
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#include <shlib-compat.h>
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#include "pthread_cond_common.c"
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/* We do the following steps from __pthread_cond_signal in one critical
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section: (1) signal all waiters in G1, (2) close G1 so that it can become
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the new G2 and make G2 the new G1, and (3) signal all waiters in the new
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G1. We don't need to do all these steps if there are no waiters in G1
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and/or G2. See __pthread_cond_signal for further details. */
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int
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___pthread_cond_broadcast (pthread_cond_t *cond)
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{
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LIBC_PROBE (cond_broadcast, 1, cond);
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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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unsigned long long int wseq = __condvar_load_wseq_relaxed (cond);
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unsigned int g2 = wseq & 1;
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unsigned int g1 = g2 ^ 1;
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wseq >>= 1;
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bool do_futex_wake = false;
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/* Step (1): signal all waiters remaining in G1. */
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if (cond->__data.__g_size[g1] != 0)
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{
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/* Add as many signals as the remaining size of the group. */
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atomic_fetch_add_relaxed (cond->__data.__g_signals + g1,
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cond->__data.__g_size[g1]);
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cond->__data.__g_size[g1] = 0;
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/* We need to wake G1 waiters before we switch G1 below. */
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/* TODO Only set it if there are indeed futex waiters. We could
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also try to move this out of the critical section in cases when
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G2 is empty (and we don't need to quiesce). */
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futex_wake (cond->__data.__g_signals + g1, INT_MAX, private);
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}
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/* G1 is complete. Step (2) is next unless there are no waiters in G2, in
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which case we can stop. */
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if (__condvar_switch_g1 (cond, wseq, &g1, private))
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{
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/* Step (3): Send signals to all waiters in the old G2 / new G1. */
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atomic_fetch_add_relaxed (cond->__data.__g_signals + g1,
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cond->__data.__g_size[g1]);
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cond->__data.__g_size[g1] = 0;
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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, INT_MAX, private);
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return 0;
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}
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versioned_symbol (libc, ___pthread_cond_broadcast,
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pthread_cond_broadcast, GLIBC_2_3_2);
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libc_hidden_ver (___pthread_cond_broadcast, __pthread_cond_broadcast)
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#ifndef SHARED
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strong_alias (___pthread_cond_broadcast, __pthread_cond_broadcast)
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#endif
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