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

[Fam Zheng <famz@redhat.com>]

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.

> 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.

Fam