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For a full analysis of both the pthread_rwlock_tryrdlock() stall and the pthread_rwlock_trywrlock() stall see: https://sourceware.org/bugzilla/show_bug.cgi?id=23844#c14 In the pthread_rwlock_trydlock() function we fail to inspect for PTHREAD_RWLOCK_FUTEX_USED in __wrphase_futex and wake the waiting readers. In the pthread_rwlock_trywrlock() function we write 1 to __wrphase_futex and loose the setting of the PTHREAD_RWLOCK_FUTEX_USED bit, again failing to wake waiting readers during unlock. The fix in the case of pthread_rwlock_trydlock() is to check for PTHREAD_RWLOCK_FUTEX_USED and wake the readers. The fix in the case of pthread_rwlock_trywrlock() is to only write 1 to __wrphase_futex if we installed the write phase, since all other readers would be spinning waiting for this step. We add two new tests, one exercises the stall for pthread_rwlock_trywrlock() which is easy to exercise, and one exercises the stall for pthread_rwlock_trydlock() which is harder to exercise. The pthread_rwlock_trywrlock() test fails consistently without the fix, and passes after. The pthread_rwlock_tryrdlock() test fails roughly 5-10% of the time without the fix, and passes all the time after. Signed-off-by: Carlos O'Donell <carlos@redhat.com> Signed-off-by: Torvald Riegel <triegel@redhat.com> Signed-off-by: Rik Prohaska <prohaska7@gmail.com> Co-authored-by: Torvald Riegel <triegel@redhat.com> Co-authored-by: Rik Prohaska <prohaska7@gmail.com>
69 lines
3.0 KiB
C
69 lines
3.0 KiB
C
/* Copyright (C) 2002-2019 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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Contributed by Ulrich Drepper <drepper@redhat.com>, 2002.
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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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<http://www.gnu.org/licenses/>. */
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#include <errno.h>
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#include "pthreadP.h"
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#include <atomic.h>
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/* See pthread_rwlock_common.c for an overview. */
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int
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__pthread_rwlock_trywrlock (pthread_rwlock_t *rwlock)
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{
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/* When in a trywrlock, we can acquire the write lock if it is in states
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#1 (idle and read phase) and #5 (idle and write phase), and also in #6
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(readers waiting, write phase) if we prefer writers.
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If we observe any other state, we are allowed to fail and do not need to
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"synchronize memory" as specified by POSIX (hence relaxed MO is
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sufficient for the first load and the CAS failure path).
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We face a similar issue as in tryrdlock in that we need to both avoid
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live-locks / starvation and must not fail spuriously (see there for
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further comments) -- and thus must loop until we get a definitive
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observation or state change. */
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unsigned int r = atomic_load_relaxed (&rwlock->__data.__readers);
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bool prefer_writer =
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(rwlock->__data.__flags != PTHREAD_RWLOCK_PREFER_READER_NP);
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while (((r & PTHREAD_RWLOCK_WRLOCKED) == 0)
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&& (((r >> PTHREAD_RWLOCK_READER_SHIFT) == 0)
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|| (prefer_writer && ((r & PTHREAD_RWLOCK_WRPHASE) != 0))))
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{
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/* Try to transition to states #7 or #8 (i.e., acquire the lock). */
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if (atomic_compare_exchange_weak_acquire (
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&rwlock->__data.__readers, &r,
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r | PTHREAD_RWLOCK_WRPHASE | PTHREAD_RWLOCK_WRLOCKED))
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{
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/* We have become the primary writer and we cannot have shared
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the PTHREAD_RWLOCK_FUTEX_USED flag with someone else, so we
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can simply enable blocking (see full wrlock code). */
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atomic_store_relaxed (&rwlock->__data.__writers_futex, 1);
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/* If we started a write phase, we need to enable readers to
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wait. If we did not, we must not change it because other threads
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may have set the PTHREAD_RWLOCK_FUTEX_USED in the meantime. */
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if ((r & PTHREAD_RWLOCK_WRPHASE) == 0)
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atomic_store_relaxed (&rwlock->__data.__wrphase_futex, 1);
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atomic_store_relaxed (&rwlock->__data.__cur_writer,
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THREAD_GETMEM (THREAD_SELF, tid));
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return 0;
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}
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/* TODO Back-off. */
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/* See above. */
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}
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return EBUSY;
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}
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strong_alias (__pthread_rwlock_trywrlock, pthread_rwlock_trywrlock)
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