__get_turnstile
struct mutex *__get_turnstile(struct mm_id *mm_id);
void enter_turnstile(struct mm_id *mm_id) __acquires(__get_turnstile(mm_id));
void exit_turnstile(struct mm_id *mm_id) __releases(__get_turnstile(mm_id));
mutex_lock(__get_turnstile(mm_id));
mutex_unlock(__get_turnstile(mm_id));