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From:   Paolo Valente <paolo.valente@linaro.org>
Message-Id: <8D356375-4C35-41EC-9F4D-0E321B08F9BF@linaro.org>
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Subject: Re: [PATCH BUGFIX 1/1] block, bfq: add requeue-request hook
Date:   Wed, 7 Feb 2018 12:27:10 +0100
In-Reply-To: <5584030F-8F4E-4C98-84A5-79873A1BF775@linaro.org>
Cc:     Oleksandr Natalenko <oleksandr@natalenko.name>,
        Jens Axboe <axboe@kernel.dk>,
        linux-block <linux-block@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Ulf Hansson <ulf.hansson@linaro.org>,
        Mark Brown <broonie@kernel.org>,
        Linus Walleij <linus.walleij@linaro.org>,
        'Paolo Valente' via bfq-iosched 
        <bfq-iosched@googlegroups.com>,
        Alban Browaeys <alban.browaeys@gmail.com>,
        Ming Lei <ming.lei@redhat.com>, Ivan Kozik <ivan@ludios.org>,
        SERENA ZIVIANI <169364@studenti.unimore.it>,
        Serena Ziviani <ziviani.serena@gmail.com>,
        =?utf-8?Q?Holger_Hoffst=C3=A4tte?= <holger@applied-asynchrony.com>
To:     Mike Galbraith <efault@gmx.de>
References: <20180205190510.5499-1-paolo.valente@linaro.org>
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> Il giorno 07 feb 2018, alle ore 11:27, Paolo Valente =
<paolo.valente@linaro.org> ha scritto:
>=20
>=20
>=20
>> Il giorno 07 feb 2018, alle ore 11:15, Mike Galbraith <efault@gmx.de> =
ha scritto:
>>=20
>> On Wed, 2018-02-07 at 10:45 +0100, Paolo Valente wrote:
>>>=20
>>>> Il giorno 07 feb 2018, alle ore 10:23, Mike Galbraith =
<efault@gmx.de> ha scritto:
>>>>=20
>>>> On Wed, 2018-02-07 at 10:08 +0100, Paolo Valente wrote:
>>>>>=20
>>>>> The first piece of information I need is whether this failure =
happens
>>>>> even without "BFQ hierarchical scheduling support".
>>>>=20
>>>> I presume you mean BFQ_GROUP_IOSCHED, which I do not have enabled.
>>>>=20
>>>=20
>>> Great (so to speak), this saves us one step.
>>>=20
>>> So, here's my next request for help: please apply the attached patch
>>> (compressed to preserve it from my email client) and retry. It adds
>>> several anomaly checks. I hope I have not added any false-positive
>>> check.
>>=20
>> kernel BUG at block/bfq-iosched.c:4742!
>>=20
>> 4742         BUG_ON(!(rq->rq_flags & RQF_ELVPRIV));
>=20
> Oh my, this is as crazy as, fortunately, easy to fix.  The problem is
> that this is easy to fix in bfq, but increases the doubts I have
> expressed in my cover letter: is it ok that, in blk-mq, the functions
> of an elevator may get invoked, without control, on requests that do
> not belong to that elevator?
>=20
> Anyway, two requests, Mike, if you haven't had enough already:
>=20
> 1. Could you paste a stack trace for this OOPS, just to understand how =
we
> get there?
>=20
> 2. Could you please turn that BUG_ON into:
> if (!(rq->rq_flags & RQF_ELVPRIV))
> 	return;
> and see what happens?
>=20

Hi Oleksandr, Holger,
before I prepare a V2 candidate patch, could you please test my
instrumentation patch too, with the above change made.  For your
convenience, I have attached a compressed archive with both the
instrumentation patch and a patch making the above change.

Crossing my fingers,
Paolo


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> Thanks a lot,
> Paolo


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