Index: channels/chan_sip.c
===================================================================
--- channels/chan_sip.c	(revision 383905)
+++ channels/chan_sip.c	(working copy)
@@ -28803,7 +28803,19 @@
 		if (res > 20) {
 			ast_debug(1, "chan_sip: ast_io_wait ran %d all at once\n", res);
 		}
-		ast_mutex_lock(&monlock);
+		res = 5;
+		while (ast_mutex_trylock(&monlock)) {
+			pthread_testcancel();
+			sched_yield();
+			--res;
+			if (res < 0) {
+				break;
+			}
+		}
+		if (res < 0) {
+			ast_debug(1, "chan_sip: ast_mutex_trylock(&monlock) has failed, not running sched this loop\n");
+			continue;
+		}
 		res = ast_sched_runq(sched);
 		if (res >= 20) {
 			ast_debug(1, "chan_sip: ast_sched_runq ran %d all at once\n", res);