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


From: Paolo Bonzini
Subject: Re: [Qemu-block] [PATCH] qed: fix bdrv_qed_drain
Date: Tue, 23 Feb 2016 14:54:02 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.0


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.



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