Re: block: silently error unsupported empty barriers too

[Mike Snitzer <snitzer@redhat.com>]

On Wed, Oct 28 2009 at  4:19pm -0400,
Andrew Patterson <andrew.patterson@hp.com> wrote:

> On Thu, 2009-08-06 at 21:50 -0400, Mikulas Patocka wrote:
> > On Thu, 6 Aug 2009, Alasdair G Kergon wrote:
> > 
> > > On Thu, Aug 06, 2009 at 12:14:17PM +0100, Mark McLoughlin wrote:
> > > > We should error all barriers, even empty barriers, on devices like
> > > > virtio_blk which don't support them.
> > >  
> > > Have you considered whether or not virtio_blk actually needs to
> > > support empty barriers?
> > > 
> > > Alasdair
> > 
> > This is only for request-based drivers, where it is the responsibility of 
> > blk-core to translate barriers. I think the empty barrier request anyway 
> > in blk_do_ordered, but with an error message. So the patch changes it to 
> > discard it early and queitly. It seems ok.
> > 
> > Mikulas
> > --
> 
> This patch seems to have been dropped.  I can reproduce the error using
> the following:
> 
>      1. Create an LVM logical volume on top of a cciss device (note that
>         the cciss driver does not support barriers)
>      2. Create an ext3 file system on top of the logical volume
>      3. Mount the file-system using -obarrier=1
>      4. Copy some files onto the file-system
>      5. Run sync (dm_flush is called)
> 
> If I apply the patch (with some munging), the 
> 
> "end_request: I/O error, dev cciss/cXdY, sector 0"
> 
> messages go away.
> 
> This is not strictly a regression, given that the problem was introduced
> in 2.6.31, but I think it should still be fixed for 2.6.32.

It was resubmitted last Friday:
http://lkml.org/lkml/2009/10/23/196

And is queued in Jens' 'for-linus' branch (meaning it should get
upstream in time for 2.6.32), see:
http://git.kernel.org/?p=linux/kernel/git/axboe/linux-2.6-block.git;a=commit;h=6cafb12dc85a5bdc722791cc5070968413264909

Mike