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