From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [PATCH 2/2] virtio: correct the memory barrier in
	virtqueue_kick_prepare()
Date: Sun, 22 Jan 2012 13:47:57 +0200
Message-ID: <20120122114757.GD27222@redhat.com>
References: <20120120081658.50747.10625.stgit@amd-6168-8-1.englab.nay.redhat.com>
	<20120120081708.50747.60241.stgit@amd-6168-8-1.englab.nay.redhat.com>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
	virtualization@lists.linux-foundation.org
To: Jason Wang <jasowang@redhat.com>
Return-path: <virtualization-bounces@lists.linux-foundation.org>
Content-Disposition: inline
In-Reply-To: <20120120081708.50747.60241.stgit@amd-6168-8-1.englab.nay.redhat.com>
List-Unsubscribe: <https://lists.linuxfoundation.org/mailman/options/virtualization>,
	<mailto:virtualization-request@lists.linux-foundation.org?subject=unsubscribe>
List-Archive: <http://lists.linuxfoundation.org/pipermail/virtualization/>
List-Post: <mailto:virtualization@lists.linux-foundation.org>
List-Help: <mailto:virtualization-request@lists.linux-foundation.org?subject=help>
List-Subscribe: <https://lists.linuxfoundation.org/mailman/listinfo/virtualization>,
	<mailto:virtualization-request@lists.linux-foundation.org?subject=subscribe>
Sender: virtualization-bounces@lists.linux-foundation.org
Errors-To: virtualization-bounces@lists.linux-foundation.org
List-Id: netdev.vger.kernel.org

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;