Re: [Qemu-devel] [Qemu-block] [PATCH 2/3] block: use BDRV_POLL_WHILE() in bdrv_rw_vmstate()

[Kevin Wolf <kwolf@redhat.com>]

Am 18.05.2017 um 10:06 hat Stefan Hajnoczi geschrieben:
> On Thu, May 18, 2017 at 8:57 AM, Kevin Wolf <kwolf@redhat.com> wrote:
> > Am 17.05.2017 um 22:16 hat Eric Blake geschrieben:
> >> On 05/17/2017 12:09 PM, Stefan Hajnoczi wrote:
> >> > Calling aio_poll() directly may have been fine previously, but this is
> >> > the future, man!
> >>
> >> lol
> >>
> >> >  The difference between an aio_poll() loop and
> >> > BDRV_POLL_WHILE() is that BDRV_POLL_WHILE() releases the AioContext
> >> > around aio_poll().
> >> >
> >> > This allows the IOThread to run fd handlers or BHs to complete the
> >> > request.  Failure to release the AioContext causes deadlocks.
> >> >
> >> > Using BDRV_POLL_WHILE() partially fixes a 'savevm' hang with -object
> >> > iothread.
> >>
> >> I'm surprised at how many separate hangs we actually had!
> >
> > How hard would it be to write some test cases for this? Dataplane has
> > a serious lack of automated testing.
> 
> And this hang doesn't even require in-flight guest I/O, so it would be
> easy to reproduce in qemu-iotests if we add an IOThread mode.

I don't think I would make it a separate mode (because people would run
only one or the other - when did you last run qcow2 v2 tests?), but just
add some test cases that make use of it during the normal -qcow2 or -raw
run.

Kevin