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

[Stefan Hajnoczi]

On Thu, Apr 13, 2017 at 6:45 AM, Paolo Bonzini <address@hidden> 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.
>
> Fam explained to me what happens, and the root cause is that bdrv_drain
> never does a release/acquire pair in this case, so the I/O thread run
> remains stuck in a callback that tries to acquire.  Ironically
> reintroducing RFifoLock would probably fix this (not 100% sure).  Oops.
>
> His solution is a bit hacky, but we will hopefully be able to revert it
> in 2.10 or whenever aio_context_acquire/release will go away.

Fam, many of us will be offline Friday and Monday due to public
holidays.  Can you work on a patch that addresses Kevin's concerns
with "[PATCH for-2.9 4/5] block: Drain BH in bdrv_drained_begin"?

I'll be officially offline too but am willing to review the patch.

Thanks,
Stefan