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