block, bfq: avoid delayed merge of async queues

Since commit 430a67f9d616 ("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: 430a67f9d616 ("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>

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index 98a42dd..7bf073e 100644 (file)
--- a/block/bfq-iosched.c
+++ b/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)) &&