Add note re deadlocks

src/hb-common.cc

@@ -293,6 +293,7 @@ hb_script_get_horizontal_direction (hb_script_t script)
  * should switch to using that insted for these too.
  */
 
+/* XXX  this can result in deadlocks because we call user callbacks */
 static hb_static_mutex_t user_data_mutex;
 
 bool

src/hb-mutex-private.hh

@@ -111,6 +111,10 @@ struct hb_static_mutex_t : hb_mutex_t
 HB_END_DECLS
 
 
+/* XXX If the finish() callbacks of items in the set recursively try to
+ * modify the set, deadlock occurs.  This needs fixing in set proper in
+ * fact. */
+
 template <typename item_t>
 struct hb_threadsafe_set_t
 {