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

[Paolo Bonzini <pbonzini@redhat.com>]

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.

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.

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.

Paolo