347 lines
9.5 KiB
C
347 lines
9.5 KiB
C
/*
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* Copyright (C) 2010 The Android Open Source Project
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* All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* * Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* * Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in
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* the documentation and/or other materials provided with the
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* distribution.
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*
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* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
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* FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
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* COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
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* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
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* BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
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* OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
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* AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
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* OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
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* OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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* SUCH DAMAGE.
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*/
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#include "pthread_internal.h"
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#include <errno.h>
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/* Technical note:
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*
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* Possible states of a read/write lock:
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*
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* - no readers and no writer (unlocked)
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* - one or more readers sharing the lock at the same time (read-locked)
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* - one writer holding the lock (write-lock)
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*
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* Additionally:
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* - trying to get the write-lock while there are any readers blocks
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* - trying to get the read-lock while there is a writer blocks
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* - a single thread can acquire the lock multiple times in the same mode
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*
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* - Posix states that behaviour is undefined it a thread tries to acquire
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* the lock in two distinct modes (e.g. write after read, or read after write).
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*
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* - This implementation tries to avoid writer starvation by making the readers
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* block as soon as there is a waiting writer on the lock. However, it cannot
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* completely eliminate it: each time the lock is unlocked, all waiting threads
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* are woken and battle for it, which one gets it depends on the kernel scheduler
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* and is semi-random.
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*
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*/
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#define __likely(cond) __builtin_expect(!!(cond), 1)
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#define __unlikely(cond) __builtin_expect(!!(cond), 0)
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#define RWLOCKATTR_DEFAULT 0
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#define RWLOCKATTR_SHARED_MASK 0x0010
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extern pthread_internal_t* __get_thread(void);
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/* Return a global kernel ID for the current thread */
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static int __get_thread_id(void)
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{
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return __get_thread()->kernel_id;
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}
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int pthread_rwlockattr_init(pthread_rwlockattr_t *attr)
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{
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if (!attr)
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return EINVAL;
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*attr = PTHREAD_PROCESS_PRIVATE;
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return 0;
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}
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int pthread_rwlockattr_destroy(pthread_rwlockattr_t *attr)
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{
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if (!attr)
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return EINVAL;
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*attr = -1;
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return 0;
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}
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int pthread_rwlockattr_setpshared(pthread_rwlockattr_t *attr, int pshared)
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{
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if (!attr)
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return EINVAL;
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switch (pshared) {
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case PTHREAD_PROCESS_PRIVATE:
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case PTHREAD_PROCESS_SHARED:
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*attr = pshared;
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return 0;
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default:
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return EINVAL;
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}
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}
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int pthread_rwlockattr_getpshared(pthread_rwlockattr_t *attr, int *pshared)
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{
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if (!attr || !pshared)
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return EINVAL;
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*pshared = *attr;
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return 0;
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}
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int pthread_rwlock_init(pthread_rwlock_t *rwlock, const pthread_rwlockattr_t *attr)
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{
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pthread_mutexattr_t* lock_attr = NULL;
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pthread_condattr_t* cond_attr = NULL;
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pthread_mutexattr_t lock_attr0;
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pthread_condattr_t cond_attr0;
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int ret;
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if (rwlock == NULL)
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return EINVAL;
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if (attr && *attr == PTHREAD_PROCESS_SHARED) {
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lock_attr = &lock_attr0;
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pthread_mutexattr_init(lock_attr);
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pthread_mutexattr_setpshared(lock_attr, PTHREAD_PROCESS_SHARED);
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cond_attr = &cond_attr0;
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pthread_condattr_init(cond_attr);
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pthread_condattr_setpshared(cond_attr, PTHREAD_PROCESS_SHARED);
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}
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ret = pthread_mutex_init(&rwlock->lock, lock_attr);
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if (ret != 0)
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return ret;
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ret = pthread_cond_init(&rwlock->cond, cond_attr);
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if (ret != 0) {
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pthread_mutex_destroy(&rwlock->lock);
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return ret;
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}
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rwlock->numLocks = 0;
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rwlock->pendingReaders = 0;
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rwlock->pendingWriters = 0;
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rwlock->writerThreadId = 0;
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return 0;
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}
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int pthread_rwlock_destroy(pthread_rwlock_t *rwlock)
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{
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int ret;
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if (rwlock == NULL)
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return EINVAL;
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if (rwlock->numLocks > 0)
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return EBUSY;
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pthread_cond_destroy(&rwlock->cond);
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pthread_mutex_destroy(&rwlock->lock);
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return 0;
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}
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/* Returns TRUE iff we can acquire a read lock. */
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static __inline__ int read_precondition(pthread_rwlock_t *rwlock, int thread_id)
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{
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/* We can't have the lock if any writer is waiting for it (writer bias).
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* This tries to avoid starvation when there are multiple readers racing.
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*/
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if (rwlock->pendingWriters > 0)
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return 0;
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/* We can have the lock if there is no writer, or if we write-own it */
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/* The second test avoids a self-dead lock in case of buggy code. */
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if (rwlock->writerThreadId == 0 || rwlock->writerThreadId == thread_id)
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return 1;
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/* Otherwise, we can't have it */
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return 0;
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}
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/* returns TRUE iff we can acquire a write lock. */
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static __inline__ int write_precondition(pthread_rwlock_t *rwlock, int thread_id)
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{
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/* We can get the lock if nobody has it */
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if (rwlock->numLocks == 0)
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return 1;
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/* Or if we already own it */
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if (rwlock->writerThreadId == thread_id)
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return 1;
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/* Otherwise, not */
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return 0;
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}
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/* This function is used to waken any waiting thread contending
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* for the lock. One of them should be able to grab it after
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* that.
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*/
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static void _pthread_rwlock_pulse(pthread_rwlock_t *rwlock)
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{
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if (rwlock->pendingReaders > 0 || rwlock->pendingWriters > 0)
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pthread_cond_broadcast(&rwlock->cond);
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}
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int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock)
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{
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return pthread_rwlock_timedrdlock(rwlock, NULL);
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}
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int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock)
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{
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int ret = 0;
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if (rwlock == NULL)
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return EINVAL;
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pthread_mutex_lock(&rwlock->lock);
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if (__unlikely(!read_precondition(rwlock, __get_thread_id())))
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ret = EBUSY;
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else
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rwlock->numLocks ++;
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pthread_mutex_unlock(&rwlock->lock);
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return ret;
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}
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int pthread_rwlock_timedrdlock(pthread_rwlock_t *rwlock, const struct timespec *abs_timeout)
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{
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int thread_id, ret = 0;
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if (rwlock == NULL)
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return EINVAL;
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pthread_mutex_lock(&rwlock->lock);
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thread_id = __get_thread_id();
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if (__unlikely(!read_precondition(rwlock, thread_id))) {
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rwlock->pendingReaders += 1;
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do {
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ret = pthread_cond_timedwait(&rwlock->cond, &rwlock->lock, abs_timeout);
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} while (ret == 0 && !read_precondition(rwlock, thread_id));
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rwlock->pendingReaders -= 1;
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if (ret != 0)
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goto EXIT;
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}
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rwlock->numLocks ++;
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EXIT:
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pthread_mutex_unlock(&rwlock->lock);
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return ret;
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}
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int pthread_rwlock_wrlock(pthread_rwlock_t *rwlock)
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{
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return pthread_rwlock_timedwrlock(rwlock, NULL);
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}
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int pthread_rwlock_trywrlock(pthread_rwlock_t *rwlock)
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{
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int thread_id, ret = 0;
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if (rwlock == NULL)
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return EINVAL;
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pthread_mutex_lock(&rwlock->lock);
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thread_id = __get_thread_id();
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if (__unlikely(!write_precondition(rwlock, thread_id))) {
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ret = EBUSY;
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} else {
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rwlock->numLocks ++;
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rwlock->writerThreadId = thread_id;
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}
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pthread_mutex_unlock(&rwlock->lock);
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return ret;
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}
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int pthread_rwlock_timedwrlock(pthread_rwlock_t *rwlock, const struct timespec *abs_timeout)
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{
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int thread_id, ret = 0;
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if (rwlock == NULL)
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return EINVAL;
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pthread_mutex_lock(&rwlock->lock);
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thread_id = __get_thread_id();
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if (__unlikely(!write_precondition(rwlock, thread_id))) {
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/* If we can't read yet, wait until the rwlock is unlocked
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* and try again. Increment pendingReaders to get the
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* cond broadcast when that happens.
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*/
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rwlock->pendingWriters += 1;
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do {
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ret = pthread_cond_timedwait(&rwlock->cond, &rwlock->lock, abs_timeout);
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} while (ret == 0 && !write_precondition(rwlock, thread_id));
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rwlock->pendingWriters -= 1;
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if (ret != 0)
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goto EXIT;
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}
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rwlock->numLocks ++;
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rwlock->writerThreadId = thread_id;
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EXIT:
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pthread_mutex_unlock(&rwlock->lock);
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return ret;
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}
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int pthread_rwlock_unlock(pthread_rwlock_t *rwlock)
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{
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int ret = 0;
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if (rwlock == NULL)
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return EINVAL;
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pthread_mutex_lock(&rwlock->lock);
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/* The lock must be held */
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if (rwlock->numLocks == 0) {
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ret = EPERM;
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goto EXIT;
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}
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/* If it has only readers, writerThreadId is 0 */
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if (rwlock->writerThreadId == 0) {
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if (--rwlock->numLocks == 0)
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_pthread_rwlock_pulse(rwlock);
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}
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/* Otherwise, it has only a single writer, which
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* must be ourselves.
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*/
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else {
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if (rwlock->writerThreadId != __get_thread_id()) {
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ret = EPERM;
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goto EXIT;
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}
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if (--rwlock->numLocks == 0) {
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rwlock->writerThreadId = 0;
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_pthread_rwlock_pulse(rwlock);
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}
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}
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EXIT:
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pthread_mutex_unlock(&rwlock->lock);
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return ret;
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}
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