linux-kernel - Re: [PATCH for-next] dm: fix missing bi

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Message-ID: <20131104152001.GA12398@redhat.com>
Date:	Mon, 4 Nov 2013 10:20:02 -0500
From:	Mike Snitzer <snitzer@...hat.com>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	Jens Axboe <axboe@...nel.dk>, Kent Overstreet <kmo@...erainc.com>,
	dm-devel@...hat.com, linux-kernel@...r.kernel.org,
	Alasdair Kergon <agk@...hat.com>
Subject: Re: [PATCH for-next] dm: fix missing bi_remaining accounting

On Mon, Nov 04 2013 at 10:06am -0500,
Mikulas Patocka <mpatocka@...hat.com> wrote:

> 
> 
> On Fri, 1 Nov 2013, Jens Axboe wrote:
> 
> > On 11/01/2013 07:59 AM, Mike Snitzer wrote:
> > > Add the missing bi_remaining increment, required by the block layer's
> > > new bio-chaining code, to both the verity and old snapshot DM targets.
> > > 
> > > Otherwise users will hit the bi_remaining <= 0 BUG_ON in bio_endio().
> > 
> > Thanks Mike, added to the mix.
> > 
> > -- 
> > Jens Axboe
> 
> Hi
> 
> This improves a little bit on the previous patch, by replacing costly 
> atomic_inc with cheap atomic_set.
> 
> 
> From: Mikulas Patocka <mpatocka@...hat.com>
> 
> dm: change atomic_inc to atomic_set(1)
> 
> There are places in dm where we save bi_endio and bi_private, set them to
> target's routine, submit the bio, from the target's bi_endio routine we
> restore bi_endio and bi_private and end the bio with bi_endio.
> 
> This causes underflow of bi_remaining, so we must restore bi_remaining
> before ending the bio from the target bi_endio routine.
> 
> The code uses atomic_inc for restoration of bi_remaining. This patch
> changes it to atomic_set(1) to avoid an interlocked instruction. In the
> target's bi_endio routine we are sure that bi_remaining is zero
> (otherwise, the bi_endio routine wouldn't be called) and there are no
> concurrent users of the bio, so we can replace atomic_inc with
> atomic_set(1).

This isn't DM-specific.  Shouldn't the other places in the tree that use
atomic_inc on bi_remaining should really be converted at the same time?
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