res_sorcery_memory_cache.c: Fix deadlock with scheduler.

A deadlock can happen when a sorcery object is being expired from the
memory cache when at the same time another object is being placed into the
memory cache.  There are a couple other variations on this theme that
could cause the deadlock.  Basically if an object is being expired from
the sorcery memory cache at the same time as another thread tries to
update the next object expiration timer the deadlock can happen.

* Add a deadlock avoidance loop in expire_objects_from_cache() to check if
someone is trying to remove the scheduler callback from the scheduler.

ASTERISK-25441 #close

Change-Id: Iec7b0bdb81a72b39477727b1535b2539ad0cf4dc

res/res_sorcery_memory_cache.c

@@ -124,6 +124,8 @@ struct sorcery_memory_cache {
 	struct ast_heap *object_heap;
 	/*! \brief Scheduler item for expiring oldest object. */
 	int expire_id;
+	/*! TRUE if trying to stop the oldest object expiration scheduler item. */
+	unsigned int del_expire:1;
 #ifdef TEST_FRAMEWORK
 	/*! \brief Variable used to indicate we should notify a test when we reach empty */
 	unsigned int cache_notify;
@@ -443,7 +445,21 @@ static int expire_objects_from_cache(const void *data)
 	struct sorcery_memory_cache *cache = (struct sorcery_memory_cache *)data;
 	struct sorcery_memory_cached_object *cached;
 
-	ao2_wrlock(cache->objects);
+	/*
+	 * We need to do deadlock avoidance between a non-scheduler thread
+	 * blocking when trying to delete the scheduled entry for this
+	 * callback because the scheduler thread is running this callback
+	 * and this callback waiting for the cache->objects container lock
+	 * that the blocked non-scheduler thread already holds.
+	 */
+	while (ao2_trywrlock(cache->objects)) {
+		if (cache->del_expire) {
+			cache->expire_id = -1;
+			ao2_ref(cache, -1);
+			return 0;
+		}
+		sched_yield();
+	}
 
 	cache->expire_id = -1;
 
@@ -488,7 +504,9 @@ static void remove_all_from_cache(struct sorcery_memory_cache *cache)
 	ao2_callback(cache->objects, OBJ_UNLINK | OBJ_NOLOCK | OBJ_NODATA | OBJ_MULTIPLE,
 		NULL, NULL);
 
+	cache->del_expire = 1;
 	AST_SCHED_DEL_UNREF(sched, cache->expire_id, ao2_ref(cache, -1));
+	cache->del_expire = 0;
 }
 
 /*!
@@ -581,18 +599,9 @@ static int schedule_cache_expiration(struct sorcery_memory_cache *cache)
 		return 0;
 	}
 
-	if (cache->expire_id != -1) {
-		/* If we can't unschedule this expiration then it is currently attempting to run,
-		 * so let it run - it just means that it'll be the one scheduling instead of us.
-		 */
-		if (ast_sched_del(sched, cache->expire_id)) {
-			return 0;
-		}
-
-		/* Since it successfully cancelled we need to drop the ref to the cache it had */
-		ao2_ref(cache, -1);
-		cache->expire_id = -1;
-	}
+	cache->del_expire = 1;
+	AST_SCHED_DEL_UNREF(sched, cache->expire_id, ao2_ref(cache, -1));
+	cache->del_expire = 0;
 
 	cached = ast_heap_peek(cache->object_heap, 1);
 	if (!cached) {