Re: [Qemu-devel] [BUG] qemu-1.1.2 [FIXED-BY] qcow2: Fix avail_sectors in cluster allocation code

[Philipp Hahn <hahn@univention.de>]

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Hello Kevin, hello Michael,

On Wednesday 12 December 2012 17:54:58 Kevin Wolf wrote:
> Am 12.12.2012 15:09, schrieb Philipp Hahn:
> > Am Mittwoch 12 Dezember 2012 14:41:49 schrieb Kevin Wolf:
> >> As you can see in the commit message of that patch I was convinced that
> >> no bug did exist in practice and this was only dangerous with respect to
> >> future changes. Therefore my first question is if you're using an
> >> unmodified upstream qemu or if some backported patches are applied to
> >> it? If it's indeed unmodified, we should probably review the code once
> >> again to understand why it makes a difference.
> >
> > This were all unmodified versions directly from git between
> > "qemu-kvm-1.1.0" and "qemu-kvm-1.2.0"
> >
> > "git checkout b7ab0fea37c15ca9e249c42c46f5c48fd1a0943c" works,
> > "git checkout b7ab0fea37c15ca9e249c42c46f5c48fd1a0943c~1" is broken.
> > "git checkout qemu-kvm-1.1.2"  is broken,
> > "git checkout qemu-kvm-1.1.2 ; git cherry-pick
> > b7ab0fea37c15ca9e249c42c46f5c48fd1a0943c"  works
>
> Ok, thanks for clarifying. Then I must have missed some interesting case
> while doing the patch.

I think I found your missing link:
After filling in "QCowL2Meta *m", that request ist queued:
  QLIST_INSERT_HEAD(&s->cluster_allocs, m, next_in_flight);
do prevent double allocating the same cluster for overlapping requests, which 
is checked in do_alloc_cluster_offset().

I guess that since the sector count was wrong, the overlap detection didn't 
work and the two concurrent write requests to the same cluster overwrote each 
other.

> Ideally we would find a sequence of qemu-io commands to reliably
> reproduce this.

You're the block guru, so I leave that to you (or anybody else who knows more 
about the working of qemu-io.) ;-)

Sincerely
Philipp
-- 
Philipp Hahn           Open Source Software Engineer      hahn@univention.de
Univention GmbH        be open.                       fon: +49 421 22 232- 0
Mary-Somerville-Str.1  D-28359 Bremen                 fax: +49 421 22 232-99
                                                   http://www.univention.de/

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