Subject: Re: [PATCH 2/4] virtio_ring: switch to use BAD_RING()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 27 Dec 2022 02:21:30 -0500

On Tue, Dec 27, 2022 at 11:51:02AM +0800, Jason Wang wrote:
> On Tue, Dec 27, 2022 at 7:36 AM Michael S. Tsirkin <[email-masked]> wrote:
> >
> > On Mon, Dec 26, 2022 at 03:49:06PM +0800, Jason Wang wrote:
> > > Switch to reuse BAD_RING() to allow common logic to be implemented in
> > > BAD_RING().
> > >
> > > Signed-off-by: Jason Wang <[email-masked]>
> > > ---
> > > Changes since V1:
> > > - switch to use BAD_RING in virtio_break_device()
> > > ---
> > >  drivers/virtio/virtio_ring.c | 8 ++++----
> > >  1 file changed, 4 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > > index 2e7689bb933b..5cfb2fa8abee 100644
> > > --- a/drivers/virtio/virtio_ring.c
> > > +++ b/drivers/virtio/virtio_ring.c
> > > @@ -58,7 +58,8 @@
> > >       do {                                                    \
> > >               dev_err(&_vq->vq.vdev->dev,                     \
> > >                       "%s:"fmt, (_vq)->vq.name, ##args);      \
> > > -             (_vq)->broken = true;                           \
> > > +             /* Pairs with READ_ONCE() in virtqueue_is_broken(). */ \
> >
> > I don't think WRITE_ONCE/READ_ONCE pair as such. Can you point
> > me at documentation of such pairing?
> 
> Introduced by:
> 
> commit 60f0779862e4ab943810187752c462e85f5fa371
> Author: Parav Pandit <[email-masked]>
> Date:   Wed Jul 21 17:26:45 2021 +0300
> 
>     virtio: Improve vq->broken access to avoid any compiler optimization
> 
> I think it might still apply here since virtqueue_is_broken() is still
> put into a loop inside wait_event().
> 
> Thanks

Oh I see. Maybe it's a response to some discussion we had at the time,
at this point I can no longer say what it meant.
But you are doing right not changing it here of course.

> >
> > > +             WRITE_ONCE((_vq)->broken, true);                       \
> > >       } while (0)
> > >  #define START_USE(vq)
> > >  #define END_USE(vq)
> > > @@ -2237,7 +2238,7 @@ bool virtqueue_notify(struct virtqueue *_vq)
> > >
> > >       /* Prod other side to tell it about changes. */
> > >       if (!vq->notify(_vq)) {
> > > -             vq->broken = true;
> > > +             BAD_RING(vq, "vq %d is broken\n", vq->vq.index);
> > >               return false;
> > >       }
> > >       return true;
> > > @@ -2786,8 +2787,7 @@ void virtio_break_device(struct virtio_device *dev)
> > >       list_for_each_entry(_vq, &dev->vqs, list) {
> > >               struct vring_virtqueue *vq = to_vvq(_vq);
> > >
> > > -             /* Pairs with READ_ONCE() in virtqueue_is_broken(). */
> > > -             WRITE_ONCE(vq->broken, true);
> > > +             BAD_RING(vq, "Device break vq %d", _vq->index);
> > >       }
> > >       spin_unlock(&dev->vqs_list_lock);
> > >  }
> > > --
> > > 2.25.1
> >