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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>
79 lines
3.4 KiB
C
79 lines
3.4 KiB
C
/* Copyright (C) 2002-2021 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 <errno.h>
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#include "pthreadP.h"
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#include <atomic.h>
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#include <shlib-compat.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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versioned_symbol (libc, ___pthread_rwlock_trywrlock,
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pthread_rwlock_trywrlock, GLIBC_2_34);
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libc_hidden_ver (___pthread_rwlock_trywrlock, __pthread_rwlock_trywrlock)
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#if OTHER_SHLIB_COMPAT (libpthread, GLIBC_2_1, GLIBC_2_34)
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compat_symbol (libpthread, ___pthread_rwlock_trywrlock,
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pthread_rwlock_trywrlock, GLIBC_2_1);
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#endif
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#if OTHER_SHLIB_COMPAT (libpthread, GLIBC_2_2, GLIBC_2_34)
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compat_symbol (libpthread, ___pthread_rwlock_trywrlock,
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__pthread_rwlock_trywrlock, GLIBC_2_2);
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#endif
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