Re: [PATCH][VHOST] fix race with guest on multi-buffer used buffer updates

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

On Thu, May 20, 2010 at 09:58:06AM -0700, David L Stevens wrote:
> [for Michael Tsirkin's vhost development git tree]
> 
> This patch fixes a race between guest and host when
> adding used buffers wraps the ring. Without it, guests
> can see partial packets before num_buffers is set in
> the vnet header.
> 
> Signed-off-by: David L Stevens <dlstevens@us.ibm.com>

Could you please explain what the race is?

> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 7f2568d..74790ab 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -1065,14 +1065,6 @@ static int __vhost_add_used_n(struct vhost_virtqueue *vq,
>  		vq_err(vq, "Failed to write used");
>  		return -EFAULT;
>  	}
> -	/* Make sure buffer is written before we update index. */
> -	smp_wmb();
> -	if (put_user(vq->last_used_idx + count, &vq->used->idx)) {
> -		vq_err(vq, "Failed to increment used idx");
> -		return -EFAULT;
> -	}
> -	if (unlikely(vq->log_used))
> -		vhost_log_used(vq, used);
>  	vq->last_used_idx += count;
>  	return 0;
>  }
> @@ -1093,7 +1085,17 @@ int vhost_add_used_n(struct vhost_virtqueue *vq, struct vring_used_elem *heads,
>  		heads += n;
>  		count -= n;
>  	}
> -	return __vhost_add_used_n(vq, heads, count);
> +	r = __vhost_add_used_n(vq, heads, count);
> +
> +	/* Make sure buffer is written before we update index. */
> +	smp_wmb();
> +	if (put_user(vq->last_used_idx, &vq->used->idx)) {
> +		vq_err(vq, "Failed to increment used idx");
> +		return -EFAULT;
> +	}
> +	if (unlikely(vq->log_used))
> +		vhost_log_used(vq, vq->used->ring + start);
> +	return r;
>  }
>  
>  /* This actually signals the guest, using eventfd. */
>