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a2f0363f81
This adds new functions for futex operations, starting with wait, abstimed_wait, reltimed_wait, wake. They add documentation and error checking according to the current draft of the Linux kernel futex manpage. Waiting with absolute or relative timeouts is split into separate functions. This allows for removing a few cases of code duplication in pthreads code, which uses absolute timeouts; also, it allows us to put platform-specific code to go from an absolute to a relative timeout into the platform-specific futex abstractions.. Futex operations that can be canceled are also split out into separate functions suffixed by "_cancelable". There are separate versions for both Linux and NaCl; while they currently differ only slightly, my expectation is that the separate versions of lowlevellock-futex.h will eventually be merged into futex-internal.h when we get to move the lll_ functions over to the new futex API.
182 lines
5.4 KiB
C
182 lines
5.4 KiB
C
/* Copyright (C) 2003-2015 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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Contributed by Martin Schwidefsky <schwidefsky@de.ibm.com>, 2003.
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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 <sysdep.h>
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#include <lowlevellock.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 <elide.h>
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#include <stdbool.h>
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/* Acquire read lock for RWLOCK. Slow path. */
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static int __attribute__((noinline))
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__pthread_rwlock_rdlock_slow (pthread_rwlock_t *rwlock)
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{
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int result = 0;
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bool wake = false;
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int futex_shared =
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rwlock->__data.__shared == LLL_PRIVATE ? FUTEX_PRIVATE : FUTEX_SHARED;
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/* Lock is taken in caller. */
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while (1)
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{
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/* Make sure we are not holding the rwlock as a writer. This is
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a deadlock situation we recognize and report. */
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if (__builtin_expect (rwlock->__data.__writer
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== THREAD_GETMEM (THREAD_SELF, tid), 0))
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{
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result = EDEADLK;
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break;
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}
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/* Remember that we are a reader. */
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if (__glibc_unlikely (++rwlock->__data.__nr_readers_queued == 0))
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{
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/* Overflow on number of queued readers. */
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--rwlock->__data.__nr_readers_queued;
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result = EAGAIN;
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break;
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}
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int waitval = rwlock->__data.__readers_wakeup;
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/* Free the lock. */
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lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
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/* Wait for the writer to finish. We do not check the return value
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because we decide how to continue based on the state of the rwlock. */
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futex_wait_simple (&rwlock->__data.__readers_wakeup, waitval,
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futex_shared);
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/* Get the lock. */
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lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
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--rwlock->__data.__nr_readers_queued;
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/* Get the rwlock if there is no writer... */
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if (rwlock->__data.__writer == 0
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/* ...and if either no writer is waiting or we prefer readers. */
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&& (!rwlock->__data.__nr_writers_queued
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|| PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
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{
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/* Increment the reader counter. Avoid overflow. */
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if (__glibc_unlikely (++rwlock->__data.__nr_readers == 0))
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{
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/* Overflow on number of readers. */
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--rwlock->__data.__nr_readers;
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result = EAGAIN;
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}
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else
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{
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LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
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/* See pthread_rwlock_rdlock. */
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if (rwlock->__data.__nr_readers == 1
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&& rwlock->__data.__nr_readers_queued > 0
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&& rwlock->__data.__nr_writers_queued > 0)
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{
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++rwlock->__data.__readers_wakeup;
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wake = true;
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}
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}
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break;
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}
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}
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/* We are done, free the lock. */
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lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
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if (wake)
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futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX, futex_shared);
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return result;
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}
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/* Fast path of acquiring read lock on RWLOCK. */
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int
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__pthread_rwlock_rdlock (pthread_rwlock_t *rwlock)
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{
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int result = 0;
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bool wake = false;
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int futex_shared =
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rwlock->__data.__shared == LLL_PRIVATE ? FUTEX_PRIVATE : FUTEX_SHARED;
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LIBC_PROBE (rdlock_entry, 1, rwlock);
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if (ELIDE_LOCK (rwlock->__data.__rwelision,
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rwlock->__data.__lock == 0
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&& rwlock->__data.__writer == 0
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&& rwlock->__data.__nr_readers == 0))
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return 0;
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/* Make sure we are alone. */
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lll_lock (rwlock->__data.__lock, rwlock->__data.__shared);
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/* Get the rwlock if there is no writer... */
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if (rwlock->__data.__writer == 0
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/* ...and if either no writer is waiting or we prefer readers. */
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&& (!rwlock->__data.__nr_writers_queued
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|| PTHREAD_RWLOCK_PREFER_READER_P (rwlock)))
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{
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/* Increment the reader counter. Avoid overflow. */
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if (__glibc_unlikely (++rwlock->__data.__nr_readers == 0))
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{
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/* Overflow on number of readers. */
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--rwlock->__data.__nr_readers;
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result = EAGAIN;
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}
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else
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{
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LIBC_PROBE (rdlock_acquire_read, 1, rwlock);
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/* If we are the first reader, and there are blocked readers and
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writers (which we don't prefer, see above), then it can be the
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case that we stole the lock from a writer that was already woken
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to acquire it. That means that we need to take over the writer's
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responsibility to wake all readers (see pthread_rwlock_unlock).
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Thus, wake all readers in this case. */
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if (rwlock->__data.__nr_readers == 1
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&& rwlock->__data.__nr_readers_queued > 0
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&& rwlock->__data.__nr_writers_queued > 0)
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{
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++rwlock->__data.__readers_wakeup;
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wake = true;
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}
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}
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/* We are done, free the lock. */
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lll_unlock (rwlock->__data.__lock, rwlock->__data.__shared);
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if (wake)
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futex_wake (&rwlock->__data.__readers_wakeup, INT_MAX, futex_shared);
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return result;
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}
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return __pthread_rwlock_rdlock_slow (rwlock);
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}
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weak_alias (__pthread_rwlock_rdlock, pthread_rwlock_rdlock)
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hidden_def (__pthread_rwlock_rdlock)
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