Subject: Re: [PATCH 2/2] virtio: correct the memory barrier in virtqueue_kick_prepare()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 22 Jan 2012 13:47:57 +0200

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 <[email-masked]>

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

Acked-by: Michael S. Tsirkin <[email-masked]>

> ---
>  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;