linux-kernel - [PATCH BUGFIX 1/4] block, bfq: move forward the getting of an extra ref in bfq_bfqq

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Message-Id: <20200321094521.85986-2-paolo.valente@linaro.org>
Date:   Sat, 21 Mar 2020 10:45:18 +0100
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        bfq-iosched@...glegroups.com, oleksandr@...alenko.name,
        rasibley@...hat.com, vkabatov@...hat.com, xzhou@...hat.com,
        jstancek@...hat.com, Paolo Valente <paolo.valente@...aro.org>,
        cki-project@...hat.com
Subject: [PATCH BUGFIX 1/4] block, bfq: move forward the getting of an extra ref in bfq_bfqq_move

Commit ecedd3d7e199 ("block, bfq: get extra ref to prevent a queue
from being freed during a group move") gets an extra reference to a
bfq_queue before possibly deactivating it (temporarily), in
bfq_bfqq_move(). This prevents the bfq_queue from disappearing before
being reactivated in its new group.

Yet, the bfq_queue may also be expired (i.e., its service may be
stopped) before the bfq_queue is deactivated. And also an expiration
may lead to a premature freeing. This commit fixes this issue by
simply moving forward the getting of the extra reference already
introduced by commit ecedd3d7e199 ("block, bfq: get extra ref to
prevent a queue from being freed during a group move").

Reported-by: cki-project@...hat.com
Tested-by: cki-project@...hat.com
Signed-off-by: Paolo Valente <paolo.valente@...aro.org>
---
 block/bfq-cgroup.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
index f0ff6654af28..9d963ed518d1 100644
--- a/block/bfq-cgroup.c
+++ b/block/bfq-cgroup.c
@@ -642,6 +642,12 @@ void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 {
 	struct bfq_entity *entity = &bfqq->entity;
 
+	/*
+	 * Get extra reference to prevent bfqq from being freed in
+	 * next possible expire or deactivate.
+	 */
+	bfqq->ref++;
+
 	/* If bfqq is empty, then bfq_bfqq_expire also invokes
 	 * bfq_del_bfqq_busy, thereby removing bfqq and its entity
 	 * from data structures related to current group. Otherwise we
@@ -652,12 +658,6 @@ void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 		bfq_bfqq_expire(bfqd, bfqd->in_service_queue,
 				false, BFQQE_PREEMPTED);
 
-	/*
-	 * get extra reference to prevent bfqq from being freed in
-	 * next possible deactivate
-	 */
-	bfqq->ref++;
-
 	if (bfq_bfqq_busy(bfqq))
 		bfq_deactivate_bfqq(bfqd, bfqq, false, false);
 	else if (entity->on_st_or_in_serv)
@@ -677,7 +677,7 @@ void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 
 	if (!bfqd->in_service_queue && !bfqd->rq_in_driver)
 		bfq_schedule_dispatch(bfqd);
-	/* release extra ref taken above */
+	/* release extra ref taken above, bfqq may happen to be freed now */
 	bfq_put_queue(bfqq);
 }
 
-- 
2.20.1