Fix race condition in the scheduler when deleting a running entry.

When scheduled tasks run, they are removed from the heap (or hashtab).
When a scheduled task is deleted, if the task can't be found in the
heap (or hashtab), an assertion is triggered. If DO_CRASH is enabled,
this assertion causes a crash.

The problem is, sometimes it just so happens that someone attempts
to delete a scheduled task at the time that it is running, leading
to a crash. This change corrects the issue by tracking which task
is currently running. If that task is attempted to be deleted,
then we mark the task, and then wait for the task to complete.
This way, we can be sure to coordinate task deletion and memory
freeing.

ASTERISK-24212
Reported by Matt Jordan

Review: https://reviewboard.asterisk.org/r/3927
........

Merged revisions 422070 from http://svn.asterisk.org/svn/asterisk/branches/12
........

Merged revisions 422071 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@422072 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/sched.c

@@ -71,6 +71,13 @@ struct sched {
 	const void *data;             /*!< Data */
 	ast_sched_cb callback;        /*!< Callback */
 	ssize_t __heap_index;
+	/*!
+	 * Used to synchronize between thread running a task and thread
+	 * attempting to delete a task
+	 */
+	ast_cond_t cond;
+	/*! Indication that a running task was deleted. */
+	unsigned int deleted:1;
 };
 
 struct sched_thread {
@@ -85,6 +92,8 @@ struct ast_sched_context {
 	unsigned int highwater;					/*!< highest count so far */
 	struct ast_heap *sched_heap;
 	struct sched_thread *sched_thread;
+	/*! The scheduled task that is currently executing */
+	struct sched *currently_executing;
 
 #ifdef SCHED_MAX_CACHE
 	AST_LIST_HEAD_NOLOCK(, sched) schedc;   /*!< Cache of unused schedule structures and how many */
@@ -208,6 +217,12 @@ struct ast_sched_context *ast_sched_context_create(void)
 	return tmp;
 }
 
+static void sched_free(struct sched *task)
+{
+	ast_cond_destroy(&task->cond);
+	ast_free(task);
+}
+
 void ast_sched_context_destroy(struct ast_sched_context *con)
 {
 	struct sched *s;
@@ -219,13 +234,13 @@ void ast_sched_context_destroy(struct ast_sched_context *con)
 
 #ifdef SCHED_MAX_CACHE
 	while ((s = AST_LIST_REMOVE_HEAD(&con->schedc, list))) {
-		ast_free(s);
+		sched_free(s);
 	}
 #endif
 
 	if (con->sched_heap) {
 		while ((s = ast_heap_pop(con->sched_heap))) {
-			ast_free(s);
+			sched_free(s);
 		}
 		ast_heap_destroy(con->sched_heap);
 		con->sched_heap = NULL;
@@ -246,11 +261,14 @@ static struct sched *sched_alloc(struct ast_sched_context *con)
 	 * to minimize the number of necessary malloc()'s
 	 */
 #ifdef SCHED_MAX_CACHE
-	if ((tmp = AST_LIST_REMOVE_HEAD(&con->schedc, list)))
+	if ((tmp = AST_LIST_REMOVE_HEAD(&con->schedc, list))) {
 		con->schedccnt--;
-	else
+	} else 
 #endif
+	{
 		tmp = ast_calloc(1, sizeof(*tmp));
+		ast_cond_init(&tmp->cond, NULL);
+	}
 
 	return tmp;
 }
@@ -268,7 +286,7 @@ static void sched_release(struct ast_sched_context *con, struct sched *tmp)
 		con->schedccnt++;
 	} else
 #endif
-		ast_free(tmp);
+		sched_free(tmp);
 }
 
 /*! \brief
@@ -451,8 +469,15 @@ int _ast_sched_del(struct ast_sched_context *con, int id, const char *file, int
 		if (!ast_heap_remove(con->sched_heap, s)) {
 			ast_log(LOG_WARNING,"sched entry %d not in the sched heap?\n", s->id);
 		}
-
 		sched_release(con, s);
+	} else if (con->currently_executing && (id == con->currently_executing->id)) {
+		s = con->currently_executing;
+		s->deleted = 1;
+		/* Wait for executing task to complete so that caller of ast_sched_del() does not
+		 * free memory out from under the task.
+		 */
+		ast_cond_wait(&s->cond, &con->lock);
+		/* Do not sched_release() here because ast_sched_runq() will do it */
 	}
 
 #ifdef DUMP_SCHEDULER
@@ -591,11 +616,14 @@ int ast_sched_runq(struct ast_sched_context *con)
 		 * should return 0.
 		 */
 
+		con->currently_executing = current;
 		ast_mutex_unlock(&con->lock);
 		res = current->callback(current->data);
 		ast_mutex_lock(&con->lock);
+		con->currently_executing = NULL;
+		ast_cond_signal(&current->cond);
 
-		if (res) {
+		if (res && !current->deleted) {
 			/*
 			 * If they return non-zero, we should schedule them to be
 			 * run again.