linux-kernel - Re: Kernel crash in icq_free_icq

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Date:	Wed, 18 Jan 2012 20:05:03 +0100
From:	Jens Axboe <axboe@...nel.dk>
To:	Vivek Goyal <vgoyal@...hat.com>
CC:	Tejun Heo <tj@...nel.org>, Shaohua Li <shaohua.li@...el.com>,
	linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: Kernel crash in icq_free_icq_rcu

On 2012-01-18 17:36, Vivek Goyal wrote:
> On Wed, Jan 18, 2012 at 05:31:06PM +0100, Jens Axboe wrote:
>> On 2012-01-18 17:24, Jens Axboe wrote:
>>> On 2012-01-18 17:09, Tejun Heo wrote:
>>>> On Wed, Jan 18, 2012 at 09:20:05AM -0500, Vivek Goyal wrote:
>>>>>> Not allocating icq if request is never going to go to elevator as elevator
>>>>>> switch was happening makes sense to me.
>>>>>>
>>>>>> I tried this patch. It went little further and crashed at a different
>>>>>> place. I think this seems to be separate merging issue Tejun is trying
>>>>>> to track down.
>>>>>
>>>>> Applied Tejun's debug patch to return early and not call into elevator
>>>>> for checking whether merge is allowed or not. Things seems to be stable
>>>>> now for me.
>>>>
>>>> Yeah, plug merge is calling into elevator code without any
>>>> synchronization, so it's bound to be broken.  Given plugging is
>>>> per-task, I don't think we really need to query elevator about merging
>>>> bio's.  The request is not on elevator and plugging is part of issuing
>>>> mechanism, not scheduling, after all.  Jens, what do you think?
>>>
>>> Hmmm. We can bypass asking the elevator, as long as we query the
>>> restrictions. Does the below, by itself, resolve the crash? If yes, let
>>> me cook up a patch splitting the elv and blk rq merging logic.
>>
>> Something like the below, completely untested.
>>
>> But thinking about this a bit while doing it, why is the IO scheduler
>> going away while we have plugged requests that are elvpriv?
> 
> Not calling ioscheduler during plug merge will allow merging of sync/async
> requests together. I guess we wouldn't want that. The only check we can
> skip in case of plug merge, is whether bio and rq beong to same task/cfqq
> or not.

It's not a huge concern. Since the IO is coming from the same task, it's
definitely related. And for the related cases, we pretty much always
want merging anyway.

> May be separate elevator functions for plug merge (without lock) and
> elevator merge (with lock) will do?

I don't think that's a good idea, just the restriction checking should
be enough.

-- 
Jens Axboe

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