Re: [Qemu-stable] [PATCH] qed: do not evict in-use L2 table cache entries

[Stefan Hajnoczi]

On Thu, Mar 01, 2012 at 05:10:57PM +0100, Kevin Wolf wrote:
> Am 27.02.2012 14:16, schrieb Stefan Hajnoczi:
> > The L2 table cache reduces QED metadata reads that would be required
> > when translating LBAs to offsets into the image file.  Since requests
> > execute in parallel it is possible to share an L2 table between multiple
> > requests.
> > 
> > There is a potential data corruption issue when an in-use L2 table is
> > evicted from the cache because the following situation occurs:
> > 
> >   1. An allocating write performs an update to L2 table "A".
> > 
> >   2. Another request needs L2 table "B" and causes table "A" to be
> >      evicted.
> > 
> >   3. A new read request needs L2 table "A" but it is not cached.
> > 
> > As a result the L2 update from #1 can overlap with the L2 fetch from #3.
> > We must avoid doing overlapping I/O requests here since the worst case
> > outcome is that the L2 fetch completes before the L2 update and yields
> > stale data.  In that case we would effectively discard the L2 update and
> > lose data clusters!
> > 
> > Thanks to Benoît Canet <address@hidden> for extensive testing
> > and debugging which lead to discovery of this bug.
> > 
> > Reported-by: Benoît Canet <address@hidden>
> > Signed-off-by: Stefan Hajnoczi <address@hidden>
> 
> Thanks, applied to the block branch.
> 
> How about a qemu-iotests case?

The test case is not ready yet.  I started writing one but it is racy
because I haven't introduced a way of controlling AIO issue/complete for
tests.  My next step is to add that.

Stefan