Stay on the safe side: Only one thread is allowed

to handle an event_base.

opal/mca/event/libevent2021/libevent/event.c

@@ -1566,14 +1566,16 @@ event_base_loop(struct event_base *base, int flags)
 	 * as we invoke user callbacks. */
 	EVBASE_ACQUIRE_LOCK(base, th_base_lock);
 
-	if (base->running_loop && 0 == warn_once) {
+	if (base->running_loop) {
+            if (0 == warn_once) {
 		event_warnx("%s: reentrant invocation.  Only one event_base_loop"
 		    " can run on each event_base at once.", __func__);
                 /********* OMPI CHANGE **********/ 
                 warn_once = 1; 
                 /*********/
-                EVBASE_RELEASE_LOCK(base, th_base_lock);
+            }
-		return -1;
+            EVBASE_RELEASE_LOCK(base, th_base_lock);
+	    return -1;
 	}
 
 	base->running_loop = 1;