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

[Stefan Hajnoczi]

On Mon, Mar 07, 2016 at 05:57:41PM +0100, Kevin Wolf wrote:
> Am 23.02.2016 um 14:54 hat Paolo Bonzini geschrieben:
> > 
> > 
> > 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?
> 
> What's the status here? It would be good to have qed not segfaulting all
> the time.

I think the timer concept itself is troublesome.  A radical approach but
limited to changing QED itself is to drop the timer and instead keep a
timestamp for the last allocating write request.  Next time a write
request (allocating or rewriting) is about to complete, do the flush and
clear the need check flag as part of the write request (if 5 seconds
have passed since the timestamp).

Because the need check flag clearing is part of the write request
completion, it will integrate properly with drain.

Stefan

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