bfq io scheduler fifo_expire_sync have no function

[Zhang Bo]

In bfq_check_fifo(...) function,  bfq_bfqq_fifo_expire(bfqq) is checked at first, and then  bfq_mark_bfqq_fifo_expire(bfqq) later. 
I think bfq_check_fifo return NULL because of bfq_bfqq_fifo_expire(bfqq) return true. So expired request is not dispatch.
Could any one explain this problem?

static struct request *bfq_check_fifo(struct bfq_queue *bfqq,
				      struct request *last)
{
	struct request *rq;

	if (bfq_bfqq_fifo_expire(bfqq)) {
		return NULL;
	}

	bfq_mark_bfqq_fifo_expire(bfqq);

	...................
}