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Re: [dm-devel] [PATCH for-next] dm: fix missing bi_remaining accounting




On Mon, 4 Nov 2013, Mike Snitzer wrote:

> On Mon, Nov 04 2013 at 10:06am -0500,
> Mikulas Patocka <mpatocka redhat 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 redhat 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?

There is no 'atomic_inc.*bi_remaining' in other drivers.

It is just in fs/bio.c in bio_chain and bio_endio_nodec where it's 
probably needed.

Mikulas


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