linux-kernel - [PATCH BUGFIX 3/6] block, bfq: get extra ref to prevent a queue from being freed during a group move

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Date:   Fri, 31 Jan 2020 10:24:06 +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,
        patdung100@...il.com, cevich@...hat.com,
        Paolo Valente <paolo.valente@...aro.org>
Subject: [PATCH BUGFIX 3/6] block, bfq: get extra ref to prevent a queue from being freed during a group move

In bfq_bfqq_move(), the bfq_queue, say Q, to be moved to a new group
may happen to be deactivated in the scheduling data structures of the
source group (and then activated in the destination group). If Q is
referred only by the data structures in the source group when the
deactivation happens, then Q is freed upon the deactivation.

This commit addresses this issue by getting an extra reference before
the possible deactivation, and releasing this extra reference after Q
has been moved.

Tested-by: Chris Evich <cevich@...hat.com>
Signed-off-by: Paolo Valente <paolo.valente@...aro.org>
---
 block/bfq-cgroup.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
index e1419edde2ec..8ab7f18ff8cb 100644
--- a/block/bfq-cgroup.c
+++ b/block/bfq-cgroup.c
@@ -651,6 +651,12 @@ 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)
@@ -670,6 +676,8 @@ 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 */
+	bfq_put_queue(bfqq);
 }
 
 /**
-- 
2.20.1