exit_turnstile
void exit_turnstile(struct mm_id *mm_id) __releases(__get_turnstile(mm_id));
exit_turnstile(mm_id);
struct semaphore exit_turnstile;
sema_init(&tb->exit_turnstile, 0);
task_barrier_signal_turnstile(&tb->exit_turnstile, tb->n);
down(&tb->exit_turnstile);