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@xxxxxxxxxx>
> ---
> 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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