freebsd-src/lib/libc_r/uthread/uthread_rwlock.c
Daniel Eischen 7d7a08f174 MFlibpthread: Add a simple work-around for deadlocking on recursive
readlocks on a rwlock while there are writers waiting.
2004-01-08 15:39:12 +00:00

380 lines
8.9 KiB
C

/*-
* Copyright (c) 1998 Alex Nash
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. 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 AUTHOR 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 AUTHOR 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.
*
* $FreeBSD$
*/
#include <errno.h>
#include <limits.h>
#include <stdlib.h>
#include "namespace.h"
#include <pthread.h>
#include "un-namespace.h"
#include "pthread_private.h"
/* maximum number of times a read lock may be obtained */
#define MAX_READ_LOCKS (INT_MAX - 1)
__weak_reference(_pthread_rwlock_destroy, pthread_rwlock_destroy);
__weak_reference(_pthread_rwlock_init, pthread_rwlock_init);
__weak_reference(_pthread_rwlock_rdlock, pthread_rwlock_rdlock);
__weak_reference(_pthread_rwlock_tryrdlock, pthread_rwlock_tryrdlock);
__weak_reference(_pthread_rwlock_trywrlock, pthread_rwlock_trywrlock);
__weak_reference(_pthread_rwlock_unlock, pthread_rwlock_unlock);
__weak_reference(_pthread_rwlock_wrlock, pthread_rwlock_wrlock);
static int init_static (pthread_rwlock_t *rwlock);
static spinlock_t static_init_lock = _SPINLOCK_INITIALIZER;
static int
init_static (pthread_rwlock_t *rwlock)
{
int ret;
_SPINLOCK(&static_init_lock);
if (*rwlock == NULL)
ret = _pthread_rwlock_init(rwlock, NULL);
else
ret = 0;
_SPINUNLOCK(&static_init_lock);
return (ret);
}
int
_pthread_rwlock_destroy (pthread_rwlock_t *rwlock)
{
int ret;
if (rwlock == NULL)
ret = EINVAL;
else {
pthread_rwlock_t prwlock;
prwlock = *rwlock;
_pthread_mutex_destroy(&prwlock->lock);
_pthread_cond_destroy(&prwlock->read_signal);
_pthread_cond_destroy(&prwlock->write_signal);
free(prwlock);
*rwlock = NULL;
ret = 0;
}
return (ret);
}
int
_pthread_rwlock_init (pthread_rwlock_t *rwlock, const pthread_rwlockattr_t *attr)
{
pthread_rwlock_t prwlock;
int ret;
/* allocate rwlock object */
prwlock = (pthread_rwlock_t)malloc(sizeof(struct pthread_rwlock));
if (prwlock == NULL)
return(ENOMEM);
/* initialize the lock */
if ((ret = _pthread_mutex_init(&prwlock->lock, NULL)) != 0)
free(prwlock);
else {
/* initialize the read condition signal */
ret = _pthread_cond_init(&prwlock->read_signal, NULL);
if (ret != 0) {
_pthread_mutex_destroy(&prwlock->lock);
free(prwlock);
} else {
/* initialize the write condition signal */
ret = _pthread_cond_init(&prwlock->write_signal, NULL);
if (ret != 0) {
_pthread_cond_destroy(&prwlock->read_signal);
_pthread_mutex_destroy(&prwlock->lock);
free(prwlock);
} else {
/* success */
prwlock->state = 0;
prwlock->blocked_writers = 0;
*rwlock = prwlock;
}
}
}
return (ret);
}
int
_pthread_rwlock_rdlock (pthread_rwlock_t *rwlock)
{
pthread_rwlock_t prwlock;
struct pthread *curthread;
int ret;
if (rwlock == NULL)
return(EINVAL);
prwlock = *rwlock;
/* check for static initialization */
if (prwlock == NULL) {
if ((ret = init_static(rwlock)) != 0)
return(ret);
prwlock = *rwlock;
}
/* grab the monitor lock */
if ((ret = _pthread_mutex_lock(&prwlock->lock)) != 0)
return(ret);
/* check lock count */
if (prwlock->state == MAX_READ_LOCKS) {
_pthread_mutex_unlock(&prwlock->lock);
return (EAGAIN);
}
curthread = _get_curthread();
if ((curthread->rdlock_count > 0) && (prwlock->state > 0)) {
/*
* To avoid having to track all the rdlocks held by
* a thread or all of the threads that hold a rdlock,
* we keep a simple count of all the rdlocks held by
* a thread. If a thread holds any rdlocks it is
* possible that it is attempting to take a recursive
* rdlock. If there are blocked writers and precedence
* is given to them, then that would result in the thread
* deadlocking. So allowing a thread to take the rdlock
* when it already has one or more rdlocks avoids the
* deadlock. I hope the reader can follow that logic ;-)
*/
; /* nothing needed */
} else {
/* give writers priority over readers */
while (prwlock->blocked_writers || prwlock->state < 0) {
ret = _pthread_cond_wait(&prwlock->read_signal,
&prwlock->lock);
if (ret != 0) {
/* can't do a whole lot if this fails */
_pthread_mutex_unlock(&prwlock->lock);
return(ret);
}
}
}
curthread->rdlock_count++;
prwlock->state++; /* indicate we are locked for reading */
/*
* Something is really wrong if this call fails. Returning
* error won't do because we've already obtained the read
* lock. Decrementing 'state' is no good because we probably
* don't have the monitor lock.
*/
_pthread_mutex_unlock(&prwlock->lock);
return (ret);
}
int
_pthread_rwlock_tryrdlock (pthread_rwlock_t *rwlock)
{
pthread_rwlock_t prwlock;
struct pthread *curthread;
int ret;
if (rwlock == NULL)
return(EINVAL);
prwlock = *rwlock;
/* check for static initialization */
if (prwlock == NULL) {
if ((ret = init_static(rwlock)) != 0)
return(ret);
prwlock = *rwlock;
}
/* grab the monitor lock */
if ((ret = _pthread_mutex_lock(&prwlock->lock)) != 0)
return(ret);
curthread = _get_curthread();
if (prwlock->state == MAX_READ_LOCKS)
ret = EAGAIN; /* too many read locks acquired */
else if ((curthread->rdlock_count > 0) && (prwlock->state > 0)) {
/* see comment for pthread_rwlock_rdlock() */
curthread->rdlock_count++;
prwlock->state++;
}
/* give writers priority over readers */
else if (prwlock->blocked_writers || prwlock->state < 0)
ret = EBUSY;
else {
prwlock->state++; /* indicate we are locked for reading */
curthread->rdlock_count++;
}
/* see the comment on this in pthread_rwlock_rdlock */
_pthread_mutex_unlock(&prwlock->lock);
return (ret);
}
int
_pthread_rwlock_trywrlock (pthread_rwlock_t *rwlock)
{
pthread_rwlock_t prwlock;
int ret;
if (rwlock == NULL)
return(EINVAL);
prwlock = *rwlock;
/* check for static initialization */
if (prwlock == NULL) {
if ((ret = init_static(rwlock)) != 0)
return(ret);
prwlock = *rwlock;
}
/* grab the monitor lock */
if ((ret = _pthread_mutex_lock(&prwlock->lock)) != 0)
return(ret);
if (prwlock->state != 0)
ret = EBUSY;
else
/* indicate we are locked for writing */
prwlock->state = -1;
/* see the comment on this in pthread_rwlock_rdlock */
_pthread_mutex_unlock(&prwlock->lock);
return (ret);
}
int
_pthread_rwlock_unlock (pthread_rwlock_t *rwlock)
{
pthread_rwlock_t prwlock;
struct pthread *curthread;
int ret;
if (rwlock == NULL)
return(EINVAL);
prwlock = *rwlock;
if (prwlock == NULL)
return(EINVAL);
/* grab the monitor lock */
if ((ret = _pthread_mutex_lock(&prwlock->lock)) != 0)
return(ret);
curthread = _get_curthread();
if (prwlock->state > 0) {
curthread->rdlock_count--;
prwlock->state--;
if (prwlock->state == 0 && prwlock->blocked_writers)
ret = _pthread_cond_signal(&prwlock->write_signal);
} else if (prwlock->state < 0) {
prwlock->state = 0;
if (prwlock->blocked_writers)
ret = _pthread_cond_signal(&prwlock->write_signal);
else
ret = _pthread_cond_broadcast(&prwlock->read_signal);
} else
ret = EINVAL;
/* see the comment on this in pthread_rwlock_rdlock */
_pthread_mutex_unlock(&prwlock->lock);
return (ret);
}
int
_pthread_rwlock_wrlock (pthread_rwlock_t *rwlock)
{
pthread_rwlock_t prwlock;
int ret;
if (rwlock == NULL)
return(EINVAL);
prwlock = *rwlock;
/* check for static initialization */
if (prwlock == NULL) {
if ((ret = init_static(rwlock)) != 0)
return(ret);
prwlock = *rwlock;
}
/* grab the monitor lock */
if ((ret = _pthread_mutex_lock(&prwlock->lock)) != 0)
return(ret);
while (prwlock->state != 0) {
prwlock->blocked_writers++;
ret = _pthread_cond_wait(&prwlock->write_signal,
&prwlock->lock);
if (ret != 0) {
prwlock->blocked_writers--;
_pthread_mutex_unlock(&prwlock->lock);
return(ret);
}
prwlock->blocked_writers--;
}
/* indicate we are locked for writing */
prwlock->state = -1;
/* see the comment on this in pthread_rwlock_rdlock */
_pthread_mutex_unlock(&prwlock->lock);
return (ret);
}