Re: [Qemu-devel] [PATCH] qed: fix bdrv_qed_drain

[Paolo Bonzini <pbonzini@redhat.com>]

On 23/02/2016 13:49, Fam Zheng wrote:
> On Tue, 02/23 11:43, Paolo Bonzini wrote:
>>
>>
>> On 23/02/2016 06:57, Fam Zheng wrote:
>>>>>> +        qed_cancel_need_check_timer(s);
>>>>>> +        qed_need_check_timer_cb(s);
>>>>>> +    }
>>>>>
>>>>> What if an allocating write is queued (the else branch case)? Its completion
>>>>> will be in bdrv_drain and it could arm the need_check_timer which is wrong.
>>>>>
>>>>> We need to drain the allocating_write_reqs queue before checking the timer.
>>>>
>>>> You're right, but how?  That's what bdrv_drain(bs) does, it's a
>>>> chicken-and-egg problem.
>>>
>>> Maybe use an aio_poll loop before the if?
>>
>> That would not change the fact that you're reimplementing bdrv_drain
>> inside bdrv_qed_drain.
> 
> But it fulfills the contract of .bdrv_drain. This is the easy way, the hard way
> would be iterating through the allocating_write_reqs list and process reqs one
> by one synchronously, which still involves aio_poll indirectly.

The easy way would be better then.

Stefan, any second opinion?

Paolo

>> Perhaps for now it's simplest to just remove the QED .bdrv_drain
>> callback, if you think this patch is not a good stopgap measure to avoid
>> the segmentation faults.
> 
> OK, I'm fine with this as a stopgap measure.
> 
>> Once the bdrv_drain rework is in, we can move the callback _after_ I/O
>> is drained on bs and before it is drained on bs->file->bs.
> 
> Sounds good.