Re: [Qemu-devel] Regression from 2.8: stuck in bdrv_drain()

[Jeff Cody]

On Thu, Apr 13, 2017 at 03:39:59PM +0100, Stefan Hajnoczi wrote:
> On Thu, Apr 13, 2017 at 01:45:55PM +0800, Paolo Bonzini wrote:
> > 
> > 
> > On 13/04/2017 09:11, Jeff Cody wrote:
> > >> It didn't make it into 2.9-rc4 because of limited time. :(
> > >>
> > >> Looks like there is no -rc5, we'll have to document this as a known 
> > >> issue.
> > >> Users should "block-job-complete/cancel" as soon as possible to avoid 
> > >> such a
> > >> hang.
> > >
> > > I'd argue for including a fix for 2.9, since this is both a regression, 
> > > and
> > > a hard lock without possible recovery short of restarting the QEMU 
> > > process.
> > 
> > It is a bit of a corner case (and jobs on I/O thread are relatively rare
> > too), so maybe it's not worth delaying 2.9.  It has been delayed already
> > quite a bit.  Another reason I think I prefer to wait is to ensure that
> > we have an entry in qemu-iotests to avoid the future regression.
> 
> I also think this does not require delaying the release:
> 
> 1. It needs to be marked as a known issue in the release notes.
> 2. Let's roll the 2.9.1 stable release within a month of 2.9.0.
> 
> If both conditions are met then very few end users will be exposed to
> the problem.  I hope libvirt will create IOThreads by default soon but
> for the time being it is not a widely used configuration.
> 

Without the fix, iothreads are not usable in 2.9.0, because a running block
job can create a deadlock by a guest-initiated reboot.  I think losing the
ability to use iothreads is enough reason to warrant a fix (especially if an
-rc5 may happen anyway).

-Jeff