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func_lock: fix multiple-channel-grant problems. 

Under contention it becomes possible that multiple channels will be told
they successfully obtained the lock, which is a bug.  Please refer

ASTERISK-29217

This introduces a couple of changes.

1.  Replaces requesters ao2 container with simple counter (we don't
    really care who is waiting for the lock, only how many).  This is
    updated undex ->mutex to prevent memory access races.
2.  Correct semantics for ast_cond_timedwait() as described in
    pthread_cond_broadcast(3P) is used (multiple threads can be released
    on a single _signal()).
3.  Module unload races are taken care of and memory properly cleaned
    up.

Change-Id: I6f68b5ec82ff25b2909daf6e4d19ca864a463e29
Signed-off-by: Jaco Kroon <jaco@uls.co.za>
This commit is contained in:
Jaco Kroon 2020-12-18 21:06:20 +02:00 committed by Joshua Colp
parent 49f625b8db
commit 3a230cc6a9
1 changed files with 58 additions and 109 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

167
funcs/func_lock.c
View File

@ -110,7 +110,6 @@ static AST_LIST_HEAD_STATIC(locklist, lock_frame);
static void lock_free(void *data);
static void lock_fixup(void *data, struct ast_channel *oldchan, struct ast_channel *newchan);
static int unloading = 0;
static pthread_t broker_tid = AST_PTHREADT_NULL;
static const struct ast_datastore_info lock_info = {
.type = "MUTEX",
@ -124,8 +123,8 @@ struct lock_frame {
ast_cond_t cond;
/*! count is needed so if a recursive mutex exits early, we know how many times to unlock it. */
unsigned int count;
/*! Container of requesters for the named lock */
struct ao2_container *requesters;
/*! Count of waiting of requesters for the named lock */
unsigned int requesters;
/*! who owns us */
struct ast_channel *owner;
/*! name of the lock */
@ -147,8 +146,11 @@ static void lock_free(void *data)
while ((clframe = AST_LIST_REMOVE_HEAD(oldlist, list))) {
/* Only unlock if we own the lock */
if (clframe->channel == clframe->lock_frame->owner) {
ast_mutex_lock(&clframe->lock_frame->mutex);
clframe->lock_frame->count = 0;
clframe->lock_frame->owner = NULL;
ast_cond_signal(&clframe->lock_frame->cond);
ast_mutex_unlock(&clframe->lock_frame->mutex);
}
ast_free(clframe);
}
@ -173,54 +175,11 @@ static void lock_fixup(void *data, struct ast_channel *oldchan, struct ast_chann
if (clframe->lock_frame->owner == oldchan) {
clframe->lock_frame->owner = newchan;
}
/* We don't move requesters, because the thread stack is different */
clframe->channel = newchan;
}
AST_LIST_UNLOCK(list);
}
static void *lock_broker(void *unused)
{
struct lock_frame *frame;
struct timespec forever = { 1000000, 0 };
for (;;) {
int found_requester = 0;
/* Test for cancel outside of the lock */
pthread_testcancel();
AST_LIST_LOCK(&locklist);
AST_LIST_TRAVERSE(&locklist, frame, entries) {
if (ao2_container_count(frame->requesters)) {
found_requester++;
ast_mutex_lock(&frame->mutex);
if (!frame->owner) {
ast_cond_signal(&frame->cond);
}
ast_mutex_unlock(&frame->mutex);
}
}
AST_LIST_UNLOCK(&locklist);
pthread_testcancel();
/* If there are no requesters, then wait for a signal */
if (!found_requester) {
nanosleep(&forever, NULL);
} else {
sched_yield();
}
}
/* Not reached */
return NULL;
}
static int ast_channel_cmp_cb(void *obj, void *arg, int flags)
{
struct ast_channel *chan = obj, *cmp_args = arg;
return strcasecmp(ast_channel_name(chan), ast_channel_name(cmp_args)) ? 0 : CMP_MATCH;
}
static int get_lock(struct ast_channel *chan, char *lockname, int trylock)
{
struct ast_datastore *lock_store = ast_channel_datastore_find(chan, &lock_info, NULL);
@ -290,17 +249,13 @@ static int get_lock(struct ast_channel *chan, char *lockname, int trylock)
AST_LIST_UNLOCK(&locklist);
return -1;
}
current->requesters = ao2_container_alloc_list(AO2_ALLOC_OPT_LOCK_MUTEX, 0,
NULL, ast_channel_cmp_cb);
if (!current->requesters) {
ast_mutex_destroy(&current->mutex);
ast_cond_destroy(&current->cond);
ast_free(current);
AST_LIST_UNLOCK(&locklist);
return -1;
}
current->requesters = 0;
AST_LIST_INSERT_TAIL(&locklist, current, entries);
}
/* Add to requester list */
ast_mutex_lock(&current->mutex);
current->requesters++;
ast_mutex_unlock(&current->mutex);
AST_LIST_UNLOCK(&locklist);
/* Found lock or created one - now find or create the corresponding link in the channel */
@ -337,44 +292,42 @@ static int get_lock(struct ast_channel *chan, char *lockname, int trylock)
* the same amount, before we'll release this one.
*/
if (current->owner == chan) {
/* We're not a requester, we already have it */
ast_mutex_lock(&current->mutex);
current->requesters--;
ast_mutex_unlock(&current->mutex);
current->count++;
return 0;
}
/* Okay, we have both frames, so now we need to try to lock.
*
* Locking order: always lock locklist first. We need the
* locklist lock because the broker thread counts whether
* there are requesters with the locklist lock held, and we
* need to hold it, so that when we send our signal, below,
* to wake up the broker thread, it definitely will see that
* a requester exists at that point in time. Otherwise, we
* could add to the requesters after it has already seen that
* that lock is unoccupied and wait forever for another signal.
*/
AST_LIST_LOCK(&locklist);
ast_mutex_lock(&current->mutex);
/* Add to requester list */
ao2_link(current->requesters, chan);
pthread_kill(broker_tid, SIGURG);
AST_LIST_UNLOCK(&locklist);
/* Wait up to three seconds from now for LOCK. */
now = ast_tvnow();
timeout.tv_sec = now.tv_sec + 3;
timeout.tv_nsec = now.tv_usec * 1000;
if (!current->owner
|| (!trylock
&& !(res = ast_cond_timedwait(&current->cond, &current->mutex, &timeout)))) {
res = 0;
ast_mutex_lock(&current->mutex);
res = 0;
while (!trylock && !res && current->owner) {
res = ast_cond_timedwait(&current->cond, &current->mutex, &timeout);
}
if (current->owner) {
/* timeout;
* trylock; or
* cond_timedwait failed.
*
* either way, we fail to obtain the lock.
*/
res = -1;
} else {
current->owner = chan;
current->count++;
} else {
res = -1;
res = 0;
}
/* Remove from requester list */
ao2_unlink(current->requesters, chan);
current->requesters--;
if (res && unloading)
ast_cond_signal(&current->cond);
ast_mutex_unlock(&current->mutex);
return res;
@ -422,7 +375,10 @@ static int unlock_read(struct ast_channel *chan, const char *cmd, char *data, ch
}
if (--clframe->lock_frame->count == 0) {
ast_mutex_lock(&clframe->lock_frame->mutex);
clframe->lock_frame->owner = NULL;
ast_cond_signal(&clframe->lock_frame->cond);
ast_mutex_unlock(&clframe->lock_frame->mutex);
}
ast_copy_string(buf, "1", len);
@ -478,33 +434,33 @@ static int unload_module(void)
/* Module flag */
unloading = 1;
AST_LIST_LOCK(&locklist);
while ((current = AST_LIST_REMOVE_HEAD(&locklist, entries))) {
/* If any locks are currently in use, then we cannot unload this module */
if (current->owner || ao2_container_count(current->requesters)) {
/* Put it back */
AST_LIST_INSERT_HEAD(&locklist, current, entries);
AST_LIST_UNLOCK(&locklist);
unloading = 0;
return -1;
}
ast_mutex_destroy(&current->mutex);
ao2_ref(current->requesters, -1);
ast_free(current);
}
/* No locks left, unregister functions */
/* Make it impossible for new requesters to be added
* NOTE: channels could already be in get_lock() */
ast_custom_function_unregister(&lock_function);
ast_custom_function_unregister(&trylock_function);
ast_custom_function_unregister(&unlock_function);
if (broker_tid != AST_PTHREADT_NULL) {
pthread_cancel(broker_tid);
pthread_kill(broker_tid, SIGURG);
pthread_join(broker_tid, NULL);
AST_LIST_LOCK(&locklist);
AST_LIST_TRAVERSE(&locklist, current, entries) {
ast_mutex_lock(&current->mutex);
while (current->owner || current->requesters) {
/* either the mutex is locked, or other parties are currently in get_lock,
* we need to wait for all of those to clear first */
ast_cond_wait(&current->cond, &current->mutex);
}
ast_mutex_unlock(&current->mutex);
/* At this point we know:
* 1. the lock has been released,
* 2. there are no requesters (nor should any be able to sneak in).
*/
ast_mutex_destroy(&current->mutex);
ast_cond_destroy(&current->cond);
ast_free(current);
}
AST_LIST_UNLOCK(&locklist);
AST_LIST_HEAD_DESTROY(&locklist);
/* At this point we can safely stop access to UNLOCK */
ast_custom_function_unregister(&unlock_function);
return 0;
}
@ -515,13 +471,6 @@ static int load_module(void)
res |= ast_custom_function_register_escalating(&trylock_function, AST_CFE_READ);
res |= ast_custom_function_register_escalating(&unlock_function, AST_CFE_READ);
if (ast_pthread_create_background(&broker_tid, NULL, lock_broker, NULL)) {
ast_log(LOG_ERROR, "Failed to start lock broker thread. Unloading func_lock module.\n");
broker_tid = AST_PTHREADT_NULL;
unload_module();
return AST_MODULE_LOAD_DECLINE;
}
return res;
}