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

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Message-ID: <20141120170016.GC7495@redhat.com>
Date:	Thu, 20 Nov 2014 19:00:16 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Petr Mladek <pmladek@...e.cz>
Cc:	Tejun Heo <tj@...nel.org>, 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, 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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