Calling Xsync from the bg thread causes deadlock issues
diff -up allegro-4.4.2/src/x/xwin.c~ allegro-4.4.2/src/x/xwin.c
--- allegro-4.4.2/src/x/xwin.c~	2009-10-17 09:05:12.000000000 +0200
+++ allegro-4.4.2/src/x/xwin.c	2014-10-29 15:17:08.381148654 +0100
@@ -2463,11 +2463,8 @@ void _xwin_private_handle_input(void)
 		   _mouse_y - (_xwin_mouse_extended_range ? _xwin.scroll_y : 0));
    }
 
-   /* Flush X-buffers.  */
-   _xwin_private_flush_buffers();
-
    /* How much events are available in the queue.  */
-   events = events_queued = XEventsQueued(_xwin.display, QueuedAlready);
+   events = events_queued = XPending(_xwin.display);
    if (events <= 0)
       return;