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

[Paolo Valente]

> Il giorno 07 feb 2018, alle ore 11:15, Mike Galbraith <efault@xxxxxx> 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@xxxxxx> 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?

Thanks a lot,
Paolo