- /* Cannot detach a thread that's already exiting */
- if (a_swap(t->exitlock, 1))
+ /* If the cas fails, detach state is either already-detached
+ * or exiting/exited, and pthread_join will trap or cleanup. */
+ if (a_cas(&t->detach_state, DT_JOINABLE, DT_DETACHED) != DT_JOINABLE)