linux-kernel - Re: [PATCH] block: mq flush: fix race between IPI handler and mq flush worker

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Message-ID: <CACVXFVNprzhEpXGKze=A4T2b-y35vXRxRTOUqpywu+Et=t37Kw@mail.gmail.com>
Date:	Tue, 20 May 2014 11:20:25 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Christoph Hellwig <hch@....de>
Cc:	Jens Axboe <axboe@...nel.dk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	stable <stable@...r.kernel.org>
Subject: Re: [PATCH] block: mq flush: fix race between IPI handler and mq
 flush worker

On Mon, May 19, 2014 at 11:18 PM, Christoph Hellwig <hch@....de> wrote:
> On Mon, May 19, 2014 at 11:05:50PM +0800, Ming Lei wrote:
>> Another simple fix is to disable ipi for flush request, but looks
>> this one should be better.
>
> I think the first thing is to bite the bullet and sort out and document
> the various unions in struct request for real.

Considered the problem can be reproduced easily and cause
data loss on kvm-guest, I hope fix can be seen in 3.15. Also
it should be backported to 3.14 because someone may
enable ctx->ipi_redirect.

Thinking it further, looks the patch is fine:

- sizeof(call_single_data) and sizeof(work_struct) is very close
and both are not small, so it is reasonable to put the two in
one union for saving request space

- the conflict on the two structures just happens with flush
requests because rq->requeue_work is only used to queue
flush requests

- in the flush sequence, the conflict won't happen on request
with data, so it only happens with completing and queueing
PREFLUSH/POSTFLUSH request.

Any comments and suggestions?


Thanks,
-- 
Ming Lei
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