linux-kernel - Re: [PATCH v2] virtio-balloon: do not call blocking ops when !TASK

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Date:	Wed, 25 Feb 2015 16:37:56 +0100
From:	Thomas Huth <thuth@...ux.vnet.ibm.com>
To:	Cornelia Huck <cornelia.huck@...ibm.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2] virtio-balloon: do not call blocking ops when
 !TASK_RUNNING

On Wed, 25 Feb 2015 16:11:27 +0100
Cornelia Huck <cornelia.huck@...ibm.com> wrote:

> On Wed, 25 Feb 2015 15:36:02 +0100
> "Michael S. Tsirkin" <mst@...hat.com> wrote:
> 
> > virtio balloon has this code:
> >         wait_event_interruptible(vb->config_change,
> >                                  (diff = towards_target(vb)) != 0
> >                                  || vb->need_stats_update
> >                                  || kthread_should_stop()
> >                                  || freezing(current));
> > 
> > Which is a problem because towards_target() call might block after
> > wait_event_interruptible sets task state to TAST_INTERRUPTIBLE, causing
> > the task_struct::state collision typical of nesting of sleeping
> > primitives
> > 
> > See also http://lwn.net/Articles/628628/ or Thomas's
> > bug report
> > http://article.gmane.org/gmane.linux.kernel.virtualization/24846
> > for a fuller explanation.
> > 
> > To fix, rewrite using wait_woken.
> > 
> > Cc: stable@...r.kernel.org
> > Reported-by: Thomas Huth <thuth@...ux.vnet.ibm.com>
> > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> > ---
> > 
> > changes from v1:
> > 	remove wait_event_interruptible
> > 	noticed by Cornelia Huck <cornelia.huck@...ibm.com>
> > 
> >  drivers/virtio/virtio_balloon.c | 19 ++++++++++++++-----
> >  1 file changed, 14 insertions(+), 5 deletions(-)
> > 
> 
> I was able to reproduce Thomas' original problem and can confirm that
> it is gone with this patch.
> 
> Reviewed-by: Cornelia Huck <cornelia.huck@...ibm.com>

Right, I just applied the patch on my system, too, and the problem is
indeed gone! Thanks for the quick fix!

Tested-by: Thomas Huth <thuth@...ux.vnet.ibm.com>

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