[PATCH] CFQ: request <-> request merging rr_list fixup

In very rare circumstances would we be pruning a merged request and at
the same time delete the implicated cfqq from the rr_list, and not readd
it when the merged request got added. This could cause io stalls until
that process issued io again.

Fix it up by putting the rr_list add handling into cfq_add_rq_rb(),
identical to how pruning is handled in cfq_del_rq_rb(). This fixes a
hang reproducible with fsx-linux.
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

block/cfq-iosched.c

@@ -456,6 +456,9 @@ static void cfq_add_rq_rb(struct request *rq)
 	 */
 	while ((__alias = elv_rb_add(&cfqq->sort_list, rq)) != NULL)
 		cfq_dispatch_insert(cfqd->queue, __alias);
+
+	if (!cfq_cfqq_on_rr(cfqq))
+		cfq_add_cfqq_rr(cfqd, cfqq);
 }
 
 static inline void
@@ -1652,9 +1655,6 @@ static void cfq_insert_request(request_queue_t *q, struct request *rq)
 
 	cfq_add_rq_rb(rq);
 
-	if (!cfq_cfqq_on_rr(cfqq))
-		cfq_add_cfqq_rr(cfqd, cfqq);
-
 	list_add_tail(&rq->queuelist, &cfqq->fifo);
 
 	cfq_rq_enqueued(cfqd, cfqq, rq);