Re: kernel BUG at drivers/virtio/virtio_ring.c:218!

[Rusty Russell <rusty@rustcorp.com.au>]

On Sunday 06 April 2008 00:53:39 Balaji Rao wrote:
> On Friday 04 April 2008 01:46:21 pm Balaji Rao wrote:
> > Hi Rusty,
> >
> > I hit a bug in virtio_ring.c:218 when I was stressing virtio_net using
> > kvm with -smp 4.
> >
> > static void vring_disable_cb(struct virtqueue *_vq)
> > {
> >         struct vring_virtqueue *vq = to_vvq(_vq);
> >
> >         START_USE(vq);
> > -->        BUG_ON(vq->vring.avail->flags & VRING_AVAIL_F_NO_INTERRUPT);
> >         vq->vring.avail->flags |= VRING_AVAIL_F_NO_INTERRUPT;
> >         END_USE(vq);
> > }
> >
> > Going through the source code, I felt that this BUG_ON is not required as
> > any CPU could race and call disable_cb when one cpu still believes that
> > its enabled. To validate my understanding, I commented out the BUG_ON and
> > everything worked perfectly well.
> >
> > I also get a lot of "Unlikely: restart svq race" on my console. Under
> > high load conditions, a race could occur very often and I'm not sure if
> > that signals a buggy situation. We could printk_ratelimit if at all we
> > need to retain it.
> >
> > If you agree, I'll send a patch to this.
>
> Christian Borntraeger CCed.

Hi Balaji,

Interesting case.... can you put a '#define DEBUG' at the top of 
drivers/virtio/virtio_ring.c and re-run?

The reason we don't simply remove that check is that interrupt bugs are nasty 
to track down, usually leading to performance problems rather than outright 
breakage.

Thanks!
Rusty.

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