block, bfq: avoid delayed merge of async queues

Since commit 430a67f9d6 ("block, bfq: merge bursts of newly-created
queues"), BFQ may schedule a merge between a newly created sync
bfq_queue, say Q2, and the last sync bfq_queue created, say Q1. To this
goal, BFQ stores the address of Q1 in the field bic->stable_merge_bfqq
of the bic associated with Q2. So, when the time for the possible merge
arrives, BFQ knows which bfq_queue to merge Q2 with. In particular,
BFQ checks for possible merges on request arrivals.

Yet the same bic may also be associated with an async bfq_queue, say
Q3. So, if a request for Q3 arrives, then the above check may happen
to be executed while the bfq_queue at hand is Q3, instead of Q2. In
this case, Q1 happens to be merged with an async bfq_queue. This is
not only a conceptual mistake, because async queues are to be kept out
of queue merging, but also a bug that leads to inconsistent states.

This commits simply filters async queues out of delayed merges.

Fixes: 430a67f9d6 ("block, bfq: merge bursts of newly-created queues")
Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
Link: https://lore.kernel.org/r/20210619140948.98712-6-paolo.valente@linaro.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-iosched.c

@@ -2718,7 +2718,13 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 	 * costly and complicated.
 	 */
 	if (unlikely(!bfqd->nonrot_with_queueing)) {
-		if (bic->stable_merge_bfqq &&
+		/*
+		 * Make sure also that bfqq is sync, because
+		 * bic->stable_merge_bfqq may point to some queue (for
+		 * stable merging) also if bic is associated with a
+		 * sync queue, but this bfqq is async
+		 */
+		if (bfq_bfqq_sync(bfqq) && bic->stable_merge_bfqq &&
 		    !bfq_bfqq_just_created(bfqq) &&
 		    time_is_before_jiffies(bfqq->split_time +
 					  msecs_to_jiffies(bfq_late_stable_merging)) &&