linux-kernel - Re: [PATCH v3] virtio_balloon: Convert "vballoon" kthread into a workqueue

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Date:	Thu, 20 Nov 2014 17:55:58 +0100
From:	Petr Mladek <pmladek@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Jeff Epler <jepler@...ythonic.net>,
	Jiri Kosina <jkosina@...e.cz>,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] virtio_balloon: Convert "vballoon" kthread into a
 workqueue

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.


> > 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
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