RE: [PATCH 1/4] virtio: Improve vq->broken access to avoid any compiler optimization

[Parav Pandit via Virtualization]

> From: Michael S. Tsirkin <m...@redhat.com>
> Sent: Monday, July 19, 2021 5:35 PM
> 
> On Mon, Jul 19, 2021 at 05:26:22AM +0000, Parav Pandit wrote:
> >
> >
> > > From: Michael S. Tsirkin <m...@redhat.com>
> > > Sent: Sunday, July 18, 2021 2:09 AM
> > >
> > > On Sat, Jul 17, 2021 at 10:42:55AM +0300, Parav Pandit wrote:
> > > > Currently vq->broken field is read by virtqueue_is_broken() in
> > > > busy loop in one context by virtnet_send_command().
> > > >
> > > > vq->broken is set to true in other process context by
> > > > virtio_break_device(). Reader and writer are accessing it without
> > > > any synchronization. This may lead to a compiler optimization
> > > > which may result to optimize reading vq->broken only once.
> > > >
> > > > Hence, force reading vq->broken on each invocation of
> > > > virtqueue_is_broken() and ensure that its update is visible.
> > > >
> > > > Fixes: e2dcdfe95c0b ("virtio: virtio_break_device() to mark all
> > > > virtqueues broken.")
> > >
> > > This is all theoretical right?
> > > virtqueue_get_buf is not inlined so compiler generally assumes any
> > > vq field can change.
> > Broken bit checking cannot rely on some other kernel API for correctness.
> > So it possibly not hitting this case now, but we shouldn't depend other APIs
> usage to ensure correctness.
> >
> > >
> > > I'm inclined to not include a Fixes
> > > tag then. And please do change subject to say "theoretical"
> > > to make that clear to people.
> > >
> > I do not have any strong opinion on fixes tag. But virtio_broken_queue()
> API should be self-contained; for that I am not sure if this just theoretical.
> > Do you mean theoretical, because we haven't hit this bug?
> 
> Because with existing code I don't believe existing compilers are clever
> enough to optimize this away.
Ok. got it. I will mention in the commit log.

> 
> > > > Signed-off-by: Parav Pandit <pa...@nvidia.com>
> > > > ---
> > > >  drivers/virtio/virtio_ring.c | 6 ++++--
> > > >  1 file changed, 4 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/drivers/virtio/virtio_ring.c
> > > > b/drivers/virtio/virtio_ring.c index 89bfe46a8a7f..7f379fe7d78d
> > > > 100644
> > > > --- a/drivers/virtio/virtio_ring.c
> > > > +++ b/drivers/virtio/virtio_ring.c
> > > > @@ -2373,7 +2373,7 @@ bool virtqueue_is_broken(struct virtqueue
> > > > *_vq) {
> > > >         struct vring_virtqueue *vq = to_vvq(_vq);
> > > >
> > > > -       return vq->broken;
> > > > +       return READ_ONCE(vq->broken);
> > > >  }
> > > >  EXPORT_SYMBOL_GPL(virtqueue_is_broken);
> > > >
> > > > @@ -2387,7 +2387,9 @@ void virtio_break_device(struct
> > > > virtio_device
> > > > *dev)
> > > >
> > > >         list_for_each_entry(_vq, &dev->vqs, list) {
> > > >                 struct vring_virtqueue *vq = to_vvq(_vq);
> > > > -               vq->broken = true;
> > > > +
> > > > +               /* Pairs with READ_ONCE() in virtqueue_is_broken(). */
> > > > +               smp_store_release(&vq->broken, true);
> > >
> > > A bit puzzled here. Why do we need release semantics here?
> > > IUC store_release does not generally pair with READ_ONCE - does it?
> > >
> > It does; paired at few places, such as,
> >
> > (a) in uverbs_main.c, default_async_file
> > (b) in netlink.c, cb_table
> > (c) fs/dcache.c i_dir_seq,
> >
> > However, in this scenario, WRITE_ONCE() is enough. So I will simplify and
> use that in v1.
> >
> >
> > > The commit log does not describe this either.
> > In commit log I mentioned that "ensure that update is visible".
> > I think a better commit log is, to say: "ensure that broken bit is written".
> 
> "is read repeatedly" maybe.

I updated it to below in v2.

" Hence, force reading vq->broken on each invocation of
virtqueue_is_broken() and also force writing it so that such update is visible 
to the readers."


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