Function bqueue_dequeue

freebsd/contrib/openzfs/module/zfs/bqueue.c:129–146

* Take the first element off of q. If there are no elements on the queue, wait * until one is put there. Return the removed element. */

 * until one is put there.  Return the removed element.
 */
void *
bqueue_dequeue(bqueue_t *q)
{
	void *ret = NULL;
	uint64_t item_size;
	mutex_enter(&q->bq_lock);
	while (q->bq_size == 0) {
		cv_wait_sig(&q->bq_pop_cv, &q->bq_lock);
	}
	ret = list_remove_head(&q->bq_list);
	ASSERT3P(ret, !=, NULL);
	item_size = obj2node(q, ret)->bqn_size;
	q->bq_size -= item_size;
	if (q->bq_size <= q->bq_maxsize - (q->bq_maxsize / q->bq_fill_fraction))
		cv_signal(&q->bq_add_cv);
	mutex_exit(&q->bq_lock);
	return (ret);
}

/*
 * Returns true if the space used is 0.