Re: btrfs deadlock in 3.5-rc3

[Josef Bacik <jbacik@fusionio.com>]

On Mon, Jun 25, 2012 at 11:38:38AM -0600, Stefan Priebe wrote:
> 
> Am 25.06.2012 16:48, schrieb Josef Bacik:
> > On Mon, Jun 25, 2012 at 08:45:02AM -0600, Stefan Priebe - Profihost AG wrote:
> >>>
> >>> Thats weird, sysrq+w should have a bunch of stacktraces but it's empty, so
> >>> unless theres a bug theres nothing blocked.  Is the box actually hung or is it
> >>> just taking forever?  Maybe try sysrq+w again to see if the one you pasted was
> >>> just a fluke?  Thanks,
> >>
> >> This one looks better:
> >> http://pastebin.com/raw.php?i=R4pztDRt
> >>
> >
> > Ok looks like you have discard turned on.
> Yes
> 
>  >  Can you turn that off and see if you
> > can still reproduce the deadlock?  If so sysrq+w again, if not then I know where
> > to look ;).  Thanks,
> without discard i can't reproduce but random write speed with ceph 
> without discard is a LOT slower (around 8000 iops/s instead of 
> 13000iops/s). So i don't know if it is discard or if i'm just not able 
> to trigger it.
> 

Ouch, what kind of drive goes faster with discard _on_?  Anyway it looks like
we're waiting for the discard to come back, so either its your drive or theres a
bug in the block layer.  Maybe try an older kernel and see if it's broken there,
and then bisect it down?  Thanks,

Josef