Re: [Qemu-block] forward of qemu bug regarding random qemu hangs at shutdown

[Kevin Wolf]

Am 07.09.2018 um 18:21 hat Marc Hartmayer geschrieben:
> On Fri, Sep 07, 2018 at 11:34 AM +0200, Kevin Wolf <address@hidden> wrote:
> > Am 06.09.2018 um 21:29 hat Christian Borntraeger geschrieben:
> >> Kevin,
> >>
> >> for reference, it seems that his bug report somehow got lost.
> >> https://bugs.launchpad.net/qemu/+bug/1788582
> >
> > That looks... interesting. The reproducer doesn't even seem to use a
> > block device, and the backtrace shows a QEMU that is just sitting in the
> > main loop waiting for events, not somewhere in the shutdown process
> > after exiting the main loop where bdrv_drain_all() would be called. I
> > fail to even come up with a theory about any connection between this and
> > commit 0f12264e7.
> >
> > I think we need more information there. Can you set a breakpoint on
> > bdrv_drain_all_begin() to see where it's even called? When I start a
> > qemu instance without a block device, the first time this is called is
> > during shutdown after the mainloop (i.e. after the place where you're
> > seeing a hang).
> 
> I can try that.
> 
> >
> > Maybe bisect within the commit that seems to cause the bug, by
> > selectively disabling some hunks in it?
> 
> If I remove the line(s)
> 
> /* Execute pending BHs first (may modify the graph) and check everything
>  * else only after the BHs have executed. */
> while (aio_poll(qemu_get_aio_context(), false));
> 
> in the function 'bdrv_drain_all_poll', then it works.

It still doesn't make sense to me why this would make any difference
without a block device (and without iothreads), but you could give this
patch series of mine a try:

[PATCH 00/14] Fix some jobs/drain/aio_poll related hangs

Amongst others, it does remove the line you quoted.

Kevin