Function turnstile_signal

freebsd/kern/subr_turnstile.c:828–873

* Pick the highest priority thread on this turnstile and put it on the * pending list. This must be called with the turnstile chain locked. */

 * pending list.  This must be called with the turnstile chain locked.
 */
int
turnstile_signal(struct turnstile *ts, int queue)
{
	struct turnstile_chain *tc __unused;
	struct thread *td;
	int empty;

	MPASS(ts != NULL);
	mtx_assert(&ts->ts_lock, MA_OWNED);
	MPASS(curthread->td_proc->p_magic == P_MAGIC);
	MPASS(ts->ts_owner == curthread || ts->ts_owner == NULL);
	MPASS(queue == TS_SHARED_QUEUE || queue == TS_EXCLUSIVE_QUEUE);

	/*
	 * Pick the highest priority thread blocked on this lock and
	 * move it to the pending list.
	 */
	td = TAILQ_FIRST(&ts->ts_blocked[queue]);
	MPASS(td->td_proc->p_magic == P_MAGIC);
	mtx_lock_spin(&td_contested_lock);
	TAILQ_REMOVE(&ts->ts_blocked[queue], td, td_lockq);
	mtx_unlock_spin(&td_contested_lock);
	TAILQ_INSERT_TAIL(&ts->ts_pending, td, td_lockq);

	/*
	 * If the turnstile is now empty, remove it from its chain and
	 * give it to the about-to-be-woken thread.  Otherwise take a
	 * turnstile from the free list and give it to the thread.
	 */
	empty = TAILQ_EMPTY(&ts->ts_blocked[TS_EXCLUSIVE_QUEUE]) &&
	    TAILQ_EMPTY(&ts->ts_blocked[TS_SHARED_QUEUE]);
	if (empty) {
		tc = TC_LOOKUP(ts->ts_lockobj);
		mtx_assert(&tc->tc_lock, MA_OWNED);
		MPASS(LIST_EMPTY(&ts->ts_free));
#ifdef TURNSTILE_PROFILING
		tc->tc_depth--;
#endif
	} else
		ts = LIST_FIRST(&ts->ts_free);
	MPASS(ts != NULL);
	LIST_REMOVE(ts, ts_hash);
	td->td_turnstile = ts;

	return (empty);
}

/*
 * Put all blocked threads on the pending list.  This must be called with