linux-kernel - Re: block: fix q->flush_rq NULL pointer crash on dm-mpath flush

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Date:	Sat, 08 Mar 2014 20:18:15 -0700
From:	Jens Axboe <axboe@...nel.dk>
To:	Mike Snitzer <snitzer@...hat.com>
CC:	Hannes Reinecke <hare@...e.de>, Mike Snitzer <snitzer@...il.com>,
	Christoph Hellwig <hch@...radead.org>,
	Jeff Moyer <jmoyer@...hat.com>, Shaohua Li <shli@...ionio.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: block: fix q->flush_rq NULL pointer crash on dm-mpath flush

On 2014-03-08 17:57, Mike Snitzer wrote:
> On Sat, Mar 08 2014 at  7:24pm -0500,
> Jens Axboe <axboe@...nel.dk> wrote:
>
>> On 2014-03-08 15:09, Mike Snitzer wrote:
>>> On Sat, Mar 08 2014 at  4:33pm -0500,
>>> Hannes Reinecke <hare@...e.de> wrote:
>>>
>>>> On 03/08/2014 07:13 PM, Mike Snitzer wrote:
>>>>>
>>>>> I'm calm.. was just a bit frustrated.  But this isn't a big deal.
>>>>> I'll make an effort to reach out to relevant people sooner when
>>>>> similar stuff is reported against recently upstreamed code.  Would be
>>>>> cool if you did the same.  I can relate to needing to have the distro
>>>>> vendor hat on (first needing to determine/answer "is this issue
>>>>> specific to our hacked distro kernel?", etc).
>>>>>
>>>> The patch I made wasn't in the context of 'recently upstreamed
>>>> code', it was due to a backport Jan Kara did for our next distro
>>>> kernels (3.12-based).
>>>
>>> "3.12-based" means nothing given all the backporting for SLES, much like
>>> "3.10-based" means nothing in the context of RHEL7.
>>>
>>> The only way this fix is applicable is in the context of "recently
>>> upstreamed code", commit 1874198 ("blk-mq: rework flush sequencing
>>> logic") went upstream for v3.14-rc3.
>>>
>>> Jens, please feel free to queue this tested fix for 3.14-rc:
>>
>> Thanks Mike, queued up.
>
> Thanks.
>
>> Also queued up the list addition reversal change.
>
> I had a look at what you queued, thing is commit 1874198 replaced code
> in blk_kick_flush() that did use list_add_tail().  So getting back to
> the way the original code  was (before 1874198) would need something
> like the following patch.
>
> But it isn't clear to me why we'd have the duality of front vs tail
> additions for flushes.  Maybe Christoph knows?

Not sure it'd even make a difference with the use case, but always tail 
would be broken. But the flushing in general is a bit of a nightmare, so 
I'd be inclined to add your full fix too, at least this late in -rc.

-- 
Jens Axboe

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