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

Mike Snitzer
2013-11-04T15:20:14Z

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?