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Subject: Re: [PATCH BUGFIX 3/6] block, bfq: get extra ref to prevent a queue
 from being freed during a group move
From:   Paolo Valente <paolo.valente@linaro.org>
In-Reply-To: <784c55c0f37a1a448c31e73e28bef6f8@natalenko.name>
Date:   Fri, 31 Jan 2020 11:41:24 +0100
Cc:     Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, bfq-iosched@googlegroups.com,
        patdung100@gmail.com, cevich@redhat.com
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Message-Id: <4D4E431C-D4B7-43DD-AB66-67ED5EA807DE@linaro.org>
References: <20200131092409.10867-1-paolo.valente@linaro.org>
 <20200131092409.10867-4-paolo.valente@linaro.org>
 <784c55c0f37a1a448c31e73e28bef6f8@natalenko.name>
To:     Oleksandr Natalenko <oleksandr@natalenko.name>
Sender: linux-kernel-owner@vger.kernel.org
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[RESENDING, BECAUSE BOUNCED OFF]

> Il giorno 31 gen 2020, alle ore 11:20, Oleksandr Natalenko =
<oleksandr@natalenko.name> ha scritto:
>=20
> Hello.
>=20
> On 31.01.2020 10:24, Paolo Valente wrote:
>> 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@redhat.com>
>> Signed-off-by: Paolo Valente <paolo.valente@linaro.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++;
>=20
> Shouldn't this be hidden under some macro (bfq_get_queue_ref(), for =
instance) and also converted from int into refcount_t?
>=20

Yeah, that's in my (long) todo list.  Unfortunately, all BFQ code
handles that ref increment in that rustic way (inherited from CFQ).
It would take a little time to fix and check all occurrences.  This
fix is probably more critical, as this bug is crashing machines in some
configurations.  But I promise I won't forget your right suggestion.


Thanks,
Paolo

>> +
>> 	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);
>> }
>> /**
>=20
> --=20
>  Oleksandr Natalenko (post-factum)