Re: [PATCH UPDATED 1/3] vhost: replace vhost_workqueue with per-vhost kthread

Michael S. Tsirkin
2010-07-26T19:57:14Z

On Mon, Jul 26, 2010 at 08:51:50PM +0200, Tejun Heo wrote:
> Hello,
>
> On 07/26/2010 06:31 PM, Michael S. Tsirkin wrote:
> >> On 07/26/2010 06:05 PM, Tejun Heo wrote:
> >>> * Placing try_to_freeze() could be a bit annoying. It shouldn't be
> >>> executed when there's a work to flush.
> >
> > BTW why is this important?
> > We could always get another work and flush right after
> > try_to_freeze, and then flush would block for a long time.
> >
> > BTW the vhost patch you sent does not do this at all.
> > I am guessing it is because our thread is not freezable?
>
> Yeap, I think so.
>
> >> * Similar issue exists for kthread_stop(). The kthread shouldn't exit
> >> while there's a work to flush (please note that kthread_worker
> >> interface allows detaching / attaching worker kthread during
> >> operation, so it should remain in consistent state with regard to
> >> flushing).
> >
> > Not sure I agree here. Users must synchronise flush and stop calls.
> > Otherwise a work might get queued after stop is called, and
> > you won't be able to flush it.
>
> For freeze, it probably is okay but for stop, I think it's better to
> keep the semantics straight forward.

What are the semantics then? What do we want stop followed
by queue and flush to do?

> It may be okay to do otherwise
> but having such oddity in generic interface is nasty and may lead to
> surprises which can be pretty difficult to track down later on. It's
> just a bit more of annoyance while writing the generic code, so...
>
> Thanks.
>
> --
> tejun