cfq-iosched: fix alias + front merge bug

There's a really rare and obscure bug in CFQ, that causes a crash in
cfq_dispatch_insert() due to rq == NULL. One example of that is seen
here:

http://lkml.org/lkml/2007/4/15/41

Neil correctly diagnosed the situation for how this can happen, read
that analysis here:

http://lkml.org/lkml/2007/4/25/57

This looks like it requires md to trigger, even though it should
potentially be possible to due with O_DIRECT (at least if you edit the
kernel and doctor some of the unplug calls).

The fix is to move the ->next_rq update to when we add a request to the
rbtree. Then we remove the possibility for a request to exist in the
rbtree code, but not have ->next_rq correctly updated.
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

block/cfq-iosched.c

@@ -462,6 +462,12 @@ static void cfq_add_rq_rb(struct request *rq)
 
 	if (!cfq_cfqq_on_rr(cfqq))
 		cfq_add_cfqq_rr(cfqd, cfqq);
+
+	/*
+	 * check if this request is a better next-serve candidate
+	 */
+	cfqq->next_rq = cfq_choose_req(cfqd, cfqq->next_rq, rq);
+	BUG_ON(!cfqq->next_rq);
 }
 
 static inline void
@@ -1622,12 +1628,6 @@ cfq_rq_enqueued(struct cfq_data *cfqd, struct cfq_queue *cfqq,
 	if (rq_is_meta(rq))
 		cfqq->meta_pending++;
 
-	/*
-	 * check if this request is a better next-serve candidate)) {
-	 */
-	cfqq->next_rq = cfq_choose_req(cfqd, cfqq->next_rq, rq);
-	BUG_ON(!cfqq->next_rq);
-
 	/*
 	 * we never wait for an async request and we don't allow preemption
 	 * of an async request. so just return early