We need no write barrier here; ref #4143

src/vm.c

@@ -1984,10 +1984,7 @@ RETRY_TRY_BLOCK:
                 while (c->eidx > ci->epos) {
                   ecall_adjust();
                 }
-                if (c->fib) {
-                  mrb_write_barrier(mrb, (struct RBasic*)c->fib);
-                }
-                mrb->c->status = MRB_FIBER_TERMINATED;
+                c->status = MRB_FIBER_TERMINATED;
                 mrb->c = c->prev;
                 c->prev = NULL;
                 goto L_RAISE;