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

["Michael S. Tsirkin" <mst@redhat.com>]

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.

I'm inclined to not include a Fixes
tag then. And please do change subject to say "theoretical"
to make that clear to people.
 
> Signed-off-by: Parav Pandit <parav@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?

The commit log does not describe this either.

>  	}
>  }
>  EXPORT_SYMBOL_GPL(virtio_break_device);
> -- 
> 2.27.0

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