On Thu, Nov 20, 2014 at 05:55:58PM +0100, Petr Mladek wrote:
> On Thu 2014-11-20 11:29:35, Tejun Heo wrote:
> > On Thu, Nov 20, 2014 at 06:26:24PM +0200, Michael S. Tsirkin wrote:
> > > On Thu, Nov 20, 2014 at 06:25:43PM +0200, Michael S. Tsirkin wrote:
> > > > On Thu, Nov 20, 2014 at 11:07:46AM -0500, Tejun Heo wrote:
> > > > > On Thu, Nov 20, 2014 at 05:03:17PM +0100, Petr Mladek wrote:
> > > > > ...
> > > > > > @@ -476,7 +460,6 @@ static void virtballoon_remove(struct virtio_device *vdev)
> > > > > > {
> > > > > > struct virtio_balloon *vb = vdev->priv;
> > > > > >
> > > > > > - kthread_stop(vb->thread);
> > > > > > remove_common(vb);
> > > > > > kfree(vb);
> > > > > > }
> > > > >
> > > > > Shouldn't the work item be flushed before removal is complete?
>
> Great catch!
>
> > > > In fact, flushing it won't help because it can requeue itself, right?
> >
> > There's cancel_work_sync() to stop the self-requeueing ones.
>
> Ah, one more problem is that remove_common(vb) calls leak_balloon()
> that queues the work if not finished. We would need to add some flag
> or variant that would disable the queuing when called here.
>
That's why Tejun suggested cancel_work_sync, IIUC it stops
the requeuing without need for extra flags.
> > > From that POV a dedicated WQ kept it simple.
> >
> > A dedicated wq doesn't do anything for that. You can't shut down a
> > workqueue with a pending work item on it. destroy_workqueue() will
> > try to drain the target wq, warn if it doesn't finish in certain
> > number of iterations and just keep trying indefinitely.
>
> I wonder if it is guaranteed that none would trigger
> stats_request() or virtballoon_changed() when virtballoon_remove() is
> being called. I guess so because the original code would fail
> otherwise. The two functions access "vb->config_change"
> and the structure is freed in virtballoon_remove() without
> any protection.
>
> I am trying to confirm this by reading the code but it is not that
> easy.
>
> Best Regards,
> Petr
It's synchronized through hardware. remove_common calls reset and
del_vqs which will prevent new interrupts.
--
MST
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