Function sleepq_signal

freebsd/kern/subr_sleepqueue.c:897–946

* Find thread sleeping on a wait channel and resume it. */

 * Find thread sleeping on a wait channel and resume it.
 */
int
sleepq_signal(const void *wchan, int flags, int pri, int queue)
{
	struct sleepqueue_chain *sc;
	struct sleepqueue *sq;
	struct threadqueue *head;
	struct thread *td, *besttd;
	int wakeup_swapper;

	CTR2(KTR_PROC, "sleepq_signal(%p, %d)", wchan, flags);
	KASSERT(wchan != NULL, ("%s: invalid NULL wait channel", __func__));
	MPASS((queue >= 0) && (queue < NR_SLEEPQS));
	sq = sleepq_lookup(wchan);
	if (sq == NULL)
		return (0);
	KASSERT(sq->sq_type == (flags & SLEEPQ_TYPE),
	    ("%s: mismatch between sleep/wakeup and cv_*", __func__));

	head = &sq->sq_blocked[queue];
	if (flags & SLEEPQ_UNFAIR) {
		/*
		 * Find the most recently sleeping thread, but try to
		 * skip threads still in process of context switch to
		 * avoid spinning on the thread lock.
		 */
		sc = SC_LOOKUP(wchan);
		besttd = TAILQ_LAST_FAST(head, thread, td_slpq);
		while (besttd->td_lock != &sc->sc_lock) {
			td = TAILQ_PREV_FAST(besttd, head, thread, td_slpq);
			if (td == NULL)
				break;
			besttd = td;
		}
	} else {
		/*
		 * Find the highest priority thread on the queue.  If there
		 * is a tie, use the thread that first appears in the queue
		 * as it has been sleeping the longest since threads are
		 * always added to the tail of sleep queues.
		 */
		besttd = td = TAILQ_FIRST(head);
		while ((td = TAILQ_NEXT(td, td_slpq)) != NULL) {
			if (td->td_priority < besttd->td_priority)
				besttd = td;
		}
	}
	MPASS(besttd != NULL);
	wakeup_swapper = sleepq_resume_thread(sq, besttd, pri, SRQ_HOLD);
	return (wakeup_swapper);
}

static bool
match_any(struct thread *td __unused)