linux-kernel - Re: [PATCH BUGFIX 1/1] block, bfq: add requeue-request hook

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Date:   Wed, 7 Feb 2018 12:27:10 +0100
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Mike Galbraith <efault@....de>
Cc:     Oleksandr Natalenko <oleksandr@...alenko.name>,
        Jens Axboe <axboe@...nel.dk>,
        linux-block <linux-block@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Mark Brown <broonie@...nel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        'Paolo Valente' via bfq-iosched 
        <bfq-iosched@...glegroups.com>,
        Alban Browaeys <alban.browaeys@...il.com>,
        Ming Lei <ming.lei@...hat.com>, Ivan Kozik <ivan@...ios.org>,
        SERENA ZIVIANI <169364@...denti.unimore.it>,
        Serena Ziviani <ziviani.serena@...il.com>,
        Holger Hoffstätte <holger@...lied-asynchrony.com>
Subject: Re: [PATCH BUGFIX 1/1] block, bfq: add requeue-request hook



> Il giorno 07 feb 2018, alle ore 11:27, Paolo Valente <paolo.valente@...aro.org> ha scritto:
> 
> 
> 
>> Il giorno 07 feb 2018, alle ore 11:15, Mike Galbraith <efault@....de> ha scritto:
>> 
>> On Wed, 2018-02-07 at 10:45 +0100, Paolo Valente wrote:
>>> 
>>>> Il giorno 07 feb 2018, alle ore 10:23, Mike Galbraith <efault@....de> ha scritto:
>>>> 
>>>> On Wed, 2018-02-07 at 10:08 +0100, Paolo Valente wrote:
>>>>> 
>>>>> The first piece of information I need is whether this failure happens
>>>>> even without "BFQ hierarchical scheduling support".
>>>> 
>>>> I presume you mean BFQ_GROUP_IOSCHED, which I do not have enabled.
>>>> 
>>> 
>>> Great (so to speak), this saves us one step.
>>> 
>>> 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.
>> 
>> kernel BUG at block/bfq-iosched.c:4742!
>> 
>> 4742         BUG_ON(!(rq->rq_flags & RQF_ELVPRIV));
> 
> 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?
> 
> Anyway, two requests, Mike, if you haven't had enough already:
> 
> 1. Could you paste a stack trace for this OOPS, just to understand how we
> get there?
> 
> 2. Could you please turn that BUG_ON into:
> if (!(rq->rq_flags & RQF_ELVPRIV))
> 	return;
> and see what happens?
> 

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


Download attachment "patches.zip" of type "application/zip" (2215 bytes)




> Thanks a lot,
> Paolo