Linux Virtualization - Feb 2015 - [PATCH v2] virtio-balloon: do not call blocking ops when !TASK_RUNNING

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 at vger.kernel.org
Reported-by: Thomas Huth <thuth at linux.vnet.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
---

changes from v1:
	remove wait_event_interruptible
	noticed by Cornelia Huck <cornelia.huck at de.ibm.com>

 drivers/virtio/virtio_balloon.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 0413157..5a6ad6d 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -29,6 +29,7 @@
 #include <linux/module.h>
 #include <linux/balloon_compaction.h>
 #include <linux/oom.h>
+#include <linux/wait.h>
 
 /*
  * Balloon device works in 4K page units.  So each page is pointed to by
@@ -334,17 +335,25 @@ static int virtballoon_oom_notify(struct notifier_block
*self,
 static int balloon(void *_vballoon)
 {
 	struct virtio_balloon *vb = _vballoon;
+	DEFINE_WAIT_FUNC(wait, woken_wake_function);
 
 	set_freezable();
 	while (!kthread_should_stop()) {
 		s64 diff;
 
 		try_to_freeze();
-		wait_event_interruptible(vb->config_change,
-					 (diff = towards_target(vb)) != 0
-					 || vb->need_stats_update
-					 || kthread_should_stop()
-					 || freezing(current));
+
+		add_wait_queue(&vb->config_change, &wait);
+		for (;;) {
+			if ((diff = towards_target(vb)) != 0 ||
+			    vb->need_stats_update ||
+			    kthread_should_stop() ||
+			    freezing(current))
+				break;
+			wait_woken(&wait, TASK_INTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
+		}
+		remove_wait_queue(&vb->config_change, &wait);
+
 		if (vb->need_stats_update)
 			stats_handle_request(vb);
 		if (diff > 0)
-- 
MST

On Wed, 25 Feb 2015 15:36:02 +0100
"Michael S. Tsirkin" <mst at redhat.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 at vger.kernel.org
> Reported-by: Thomas Huth <thuth at linux.vnet.ibm.com>
> Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
> ---
> 
> changes from v1:
> 	remove wait_event_interruptible
> 	noticed by Cornelia Huck <cornelia.huck at de.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 at de.ibm.com>

On Wed, 25 Feb 2015 16:11:27 +0100
Cornelia Huck <cornelia.huck at de.ibm.com> wrote:
> On Wed, 25 Feb 2015 15:36:02 +0100
> "Michael S. Tsirkin" <mst at redhat.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 at vger.kernel.org
> > Reported-by: Thomas Huth <thuth at linux.vnet.ibm.com>
> > Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
> > ---
> > 
> > changes from v1:
> > 	remove wait_event_interruptible
> > 	noticed by Cornelia Huck <cornelia.huck at de.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 at de.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 at linux.vnet.ibm.com>