[PATCH] Fix cfq_find_next_crq()

In cfq_find_next_crq(), cfq tries to find the next request by choosing
one of two requests before and after the current one.  Currently, when
choosing the next request, if there's no next request, the next
candidate is NULL, resulting in selection of the previous request.  This
results in weird scheduling.  Once we reach the end, we always seek
backward.

The correct behavior is using the first request as the next candidate.
cfq_choose_req() already has logics for handling wrapped requests.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jens Axboe <axboe@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/block/cfq-iosched.c

@@ -375,9 +375,10 @@ cfq_find_next_crq(struct cfq_data *cfqd, struct cfq_queue *cfqq,
 	struct cfq_rq *crq_next = NULL, *crq_prev = NULL;
 	struct rb_node *rbnext, *rbprev;
 
+	rbnext = NULL;
 	if (ON_RB(&last->rb_node))
 		rbnext = rb_next(&last->rb_node);
-	else {
+	if (!rbnext) {
 		rbnext = rb_first(&cfqq->sort_list);
 		if (rbnext == &last->rb_node)
 			rbnext = NULL;