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

[Marc Hartmayer]

On Mon, Sep 10, 2018 at 10:21 AM +0200, Kevin Wolf <address@hidden> wrote:
> 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.

With the patches applied, I can no longer reproduce the bug.

>
> Kevin
>
--
Kind regards / Beste Grüße
   Marc Hartmayer

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