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

[John Snow]

On 04/13/2017 11:02 AM, Jeff Cody wrote:
> 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
> 

Not that it's my area of expertise, but given that Fam's "hacky" patch
fixes two issues now and this is a deadlock that may indeed occur
through normal usage, I'd recommend it go into an rc5 if we're spinning
one anyway.

+1 to Jeff's reasoning.

--js