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

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Message-ID: <20150225161127.51f06c8c.cornelia.huck@de.ibm.com>
Date:	Wed, 25 Feb 2015 16:11:27 +0100
From:	Cornelia Huck <cornelia.huck@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	Thomas Huth <thuth@...ux.vnet.ibm.com>,
	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 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>

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