linux-kernel - Re: blk-mq flush fix

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Message-ID: <526E90D7.1070604@kernel.dk>
Date:	Mon, 28 Oct 2013 10:29:11 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Christoph Hellwig <hch@...radead.org>
CC:	Alexander Gordeev <agordeev@...hat.com>, Tejun Heo <tj@...nel.org>,
	Nicholas Bellinger <nab@...ux-iscsi.org>,
	linux-kernel@...r.kernel.org
Subject: Re: blk-mq flush fix

On 10/28/2013 02:48 AM, Christoph Hellwig wrote:
> On Sun, Oct 27, 2013 at 10:29:25PM +0000, Jens Axboe wrote:
>> On Sat, Oct 26 2013, Christoph Hellwig wrote:
>>> I think this variant of the patch from Alexander should fix the issue
>>> in a minimally invasive way.  Longer term I'd prefer to use q->flush_rq
>>> like in the non-mq case by copying over the context and tag information.
>>
>> This one is pretty simple, we could definitely use it as a band aid. I
>> too would greatly prefer using the static ->flush_rq instead. Just have
>> it marked to bypass most of the free logic.
> 
> We already bypass the free logical by setting and end_io callback for
> a while, similar to what the old code does.  Maybe it's not all that
> hard to prealloc the request, let me give a sping.  Using the static
> allocated one will be hard due to the driver-specific extra data,
> though.

It's not that I think the existing patch is THAT bad, it fits in alright
with the reserved tagging and works regardless of whether a driver uses
reserved tags or not. And it does have the upside of not requiring
special checks or logic for this special non-tagged request that using
the preallocated would might need.

>> I'll add this one.
> 
> Gimme another day or so to figure this out.

OK, holding off.

-- 
Jens Axboe

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