skip machine stack marking for current ec.

* vm.c (rb_execution_context_mark): At the beggining of GC marking,
  mark_current_machine_context() marks running machine stack so that
  rb_execution_context_mark() should not mark machine stack if it is
  current ec.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60499 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

vm.c

@@ -2398,7 +2398,9 @@ rb_execution_context_mark(const rb_execution_context_t *ec)
     }
 
     /* mark machine stack */
-    if (ec->machine.stack_start && ec->machine.stack_end) {
+    if (ec->machine.stack_start && ec->machine.stack_end &&
+	ec != GET_EC() /* marked for current ec at the first stage of marking */
+	) {
 	rb_gc_mark_machine_stack(ec);
 	rb_gc_mark_locations((VALUE *)&ec->machine.regs,
 			     (VALUE *)(&ec->machine.regs) +