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SubjectRe: [PATCH 2/2] virtio: correct the memory barrier in virtqueue_kick_prepare()
On Fri, Jan 20, 2012 at 04:17:08PM +0800, Jason Wang wrote:
> Use virtio_mb() to make sure the available index to be exposed before
> checking the the avail event. Otherwise we may get stale value of
> avail event in guest and never kick the host after.
>
> Signed-off-by: Jason Wang <jasowang@redhat.com>

Good catch.
Note: this fixes a bug introduced by ee7cd8981e15bcb365fc762afe3fc47b8242f630.

Acked-by: Michael S. Tsirkin <mst@redhat.com>

> ---
> drivers/virtio/virtio_ring.c | 6 +++---
> 1 files changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 78428a8..07d9360 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -308,9 +308,9 @@ bool virtqueue_kick_prepare(struct virtqueue *_vq)
> bool needs_kick;
>
> START_USE(vq);
> - /* Descriptors and available array need to be set before we expose the
> - * new available array entries. */
> - virtio_wmb(vq);
> + /* We need expose available array entries before checking avail

Nit:
We need expose -> Need to expose

> + * event. */
> + virtio_mb(vq);
>
> old = vq->vring.avail->idx - vq->num_added;
> new = vq->vring.avail->idx;


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