linux-kernel - [PATCH 5.13 373/800] block, bfq: avoid delayed merge of async queues

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Date:   Mon, 12 Jul 2021 08:06:36 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Holger Hoffstätte 
        <holger@...lied-asynchrony.com>,
        Paolo Valente <paolo.valente@...aro.org>,
        Jens Axboe <axboe@...nel.dk>, Sasha Levin <sashal@...nel.org>
Subject: [PATCH 5.13 373/800] block, bfq: avoid delayed merge of async queues

From: Paolo Valente <paolo.valente@...aro.org>

[ Upstream commit bd3664b362381c4c1473753ebedf0ab242a60d1d ]

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@...lied-asynchrony.com>
Signed-off-by: Paolo Valente <paolo.valente@...aro.org>
Link: https://lore.kernel.org/r/20210619140948.98712-6-paolo.valente@linaro.org
Signed-off-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 block/bfq-iosched.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index d00c89f6ba59..7c62bf093199 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -2703,7 +2703,13 @@ 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(200))) {
-- 
2.30.2