Re: [Qemu-devel] [PATCH v9 06/10] replay: fix save/load vm for non-empty queue

[Paolo Bonzini]

On 04/05/2017 14:34, Pavel Dovgalyuk wrote:
>> From: Paolo Bonzini [mailto:address@hidden
>> On 04/05/2017 14:10, Pavel Dovgalyuk wrote:
>>>> From: Paolo Bonzini [mailto:address@hidden
>>>> On 04/05/2017 13:54, Pavel Dovgalyuk wrote:
>>>>>> From: Paolo Bonzini [mailto:address@hidden
>>>>>> On 04/05/2017 13:13, Pavel Dovgalyuk wrote:
>>>>>>>>> This patch does not allows saving/loading vmstate when
>>>>>>>>> replay events queue is not empty. There is no reliable
>>>>>>>>> way to save events queue, because it describes internal
>>>>>>>>> coroutine state. Therefore saving and loading operations
>>>>>>>>> should be deferred to another record/replay step.
>>>>>>>>
>>>>>>>> Can it actually be non-empty after bdrv_drain_all?
>>>>>>>
>>>>>>> drain/flush cannot succeed, because started requests are
>>>>>>> prisoned in the replay events queue.
>>>>>>
>>>>>> But that would apply to loading only.  Saving should still be always
>>>>>> possible.
>>>>>
>>>>> We can save it. But it wouldn't load correctly - replay queue will be 
>>>>> empty after loading.
>>>>
>>>> When saving you can drain, and then the events queue should be empty.
>>>> Or I am misunderstanding how it works, which is possible too.
>>>
>>> Drain will wait until the queue becomes empty.
>>> Queue is processed only at checkpoints.
>>> Checkpoints are met in iothread (at timers processing and so on).
>>> But iothread is waiting for finishing the drain.
>>
>> What checkpoint can hang the drain during a save?
> 
> No one. There are no checkpoins during vmsave and during drain.
> Therefore the queue will never become empty.

Understood.  And what checkpoint will you be waiting for during drain,
causing a deadlock?

Paolo