sched: Don't allow ast_sched_del to deadlock ast_sched_runq from same thread

When fixing ASTERISK~24212, a change was done so a scheduled callback could not
be removed while it was running. The caller of ast_sched_del would have to wait.

However, when the caller of ast_sched_del is the callback itself (however wrong
this might be), this new check would cause a deadlock: it would wait forever
for itself.

This changeset introduces an additional check: if ast_sched_del is called
by the callback itself, it is immediately rejected (along with an ERROR log and
a backtrace). Additionally, the AST_SCHED_DEL_UNREF macro is adjusted so the
after-ast_sched_del-refcall function is only run if ast_sched_del returned
success.

This should fix the following spurious race condition found in chan_sip:
- thread 1: schedule sip_poke_peer_now (using AST_SCHED_REPLACE)
- thread 2: run sip_poke_peer_now
- thread 2: blank out sched-ID (too soon!)
- thread 1: set sched-ID (too late!)
- thread 2: try to delete the currently running sched-ID

After this fix, an ERROR would be logged, but no deadlocks (in do_monitor) nor
excess calls to sip_unref_peer(peer) (causing double frees of rtp_instances and
other madness) should occur.

(Thanks Richard Mudgett for reviewing/improving this "scary" change.)

Note that this change does not fix the observed race condition: unlocked
access to peer->pokeexpire (and potentially other scheduled items in chan_sip),
causing AST_SCHED_DEL_UNREF to look at a changing id. But it will make the
deadlock go away. And in the observed case, it will not have adverse affects
(like memory leaks) because the scheduled item is removed through a different
path.

ASTERISK-28282

Change-Id: Ic26777fa0732725e6ca7010df17af77a012aa856

include/asterisk/sched.h

@@ -71,20 +71,24 @@ extern "C" {
 
 /*!
  * \brief schedule task to get deleted and call unref function
+ *
+ * Only calls unref function if the delete succeeded.
+ *
  * \sa AST_SCHED_DEL
  * \since 1.6.1
  */
 #define AST_SCHED_DEL_UNREF(sched, id, refcall)			\
 	do { \
-		int _count = 0; \
-		while (id > -1 && ast_sched_del(sched, id) && ++_count < 10) { \
+		int _count = 0, _id; \
+		while ((_id = id) > -1 && ast_sched_del(sched, _id) && ++_count < 10) { \
 			usleep(1); \
 		} \
-		if (_count == 10) \
-			ast_log(LOG_WARNING, "Unable to cancel schedule ID %d.  This is probably a bug (%s: %s, line %d).\n", id, __FILE__, __PRETTY_FUNCTION__, __LINE__); \
-		if (id > -1) \
+		if (_count == 10) { \
+			ast_log(LOG_WARNING, "Unable to cancel schedule ID %d.  This is probably a bug (%s: %s, line %d).\n", _id, __FILE__, __PRETTY_FUNCTION__, __LINE__); \
+		} else if (_id > -1) { \
 			refcall; \
-		id = -1; \
+			id = -1; \
+		} \
 	} while (0);
 
 /*!

main/sched.c

@@ -116,6 +116,8 @@ struct ast_sched_context {
 	struct sched_thread *sched_thread;
 	/*! The scheduled task that is currently executing */
 	struct sched *currently_executing;
+	/*! Valid while currently_executing is not NULL */
+	pthread_t executing_thread_id;
 
 #ifdef SCHED_MAX_CACHE
 	AST_LIST_HEAD_NOLOCK(, sched) schedc;   /*!< Cache of unused schedule structures and how many */
@@ -625,15 +627,26 @@ int ast_sched_del(struct ast_sched_context *con, int id)
 		}
 		sched_release(con, s);
 	} else if (con->currently_executing && (id == con->currently_executing->sched_id->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.
-		 */
-		while (con->currently_executing && (id == con->currently_executing->sched_id->id)) {
-			ast_cond_wait(&s->cond, &con->lock);
+		if (con->executing_thread_id == pthread_self()) {
+			/* The scheduled callback is trying to delete itself.
+			 * Not good as that is a deadlock. */
+			ast_log(LOG_ERROR,
+				"BUG! Trying to delete sched %d from within the callback %p.  "
+				"Ignoring so we don't deadlock\n",
+				id, con->currently_executing->callback);
+			ast_log_backtrace();
+			/* We'll return -1 below because s is NULL.
+			 * The caller will rightly assume that the unscheduling failed. */
+		} else {
+			s = con->currently_executing;
+			s->deleted = 1;
+			/* Wait for executing task to complete so that the caller of
+			 * ast_sched_del() does not free memory out from under the task. */
+			while (con->currently_executing && (id == con->currently_executing->sched_id->id)) {
+				ast_cond_wait(&s->cond, &con->lock);
+			}
+			/* Do not sched_release() here because ast_sched_runq() will do it */
 		}
-		/* Do not sched_release() here because ast_sched_runq() will do it */
 	}
 
 #ifdef DUMP_SCHEDULER
@@ -773,6 +786,7 @@ int ast_sched_runq(struct ast_sched_context *con)
 		 */
 
 		con->currently_executing = current;
+		con->executing_thread_id = pthread_self();
 		ast_mutex_unlock(&con->lock);
 		res = current->callback(current->data);
 		ast_mutex_lock(&con->lock);