Subject: Re: [PATCH 5/6] vhost_net: poll vhost queue after marking DMA is done

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 16 Aug 2013 13:00:55 +0300

On Fri, Aug 16, 2013 at 01:16:29PM +0800, Jason Wang wrote:
> We used to poll vhost queue before making DMA is done, this is racy if vhost
> thread were waked up before marking DMA is done which can result the signal to
> be missed. Fix this by always poll the vhost thread before DMA is done.
> 
> Signed-off-by: Jason Wang <[email-masked]>

Indeed, but vhost_net_ubuf_put should be the last thing we do:
it can cause the device to go away and we'll get
a user after free.

> ---
>  drivers/vhost/net.c |    9 +++++----
>  1 files changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 70cab75..a035a89 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -308,6 +308,11 @@ static void vhost_zerocopy_callback(struct ubuf_info *ubuf, bool success)
>  	struct vhost_virtqueue *vq = ubufs->vq;
>  	int cnt = atomic_read(&ubufs->kref.refcount);
>  
> +	/* set len to mark this desc buffers done DMA */
> +	vq->heads[ubuf->desc].len = success ?
> +		VHOST_DMA_DONE_LEN : VHOST_DMA_FAILED_LEN;
> +	vhost_net_ubuf_put(ubufs);
> +
>  	/*
>  	 * Trigger polling thread if guest stopped submitting new buffers:
>  	 * in this case, the refcount after decrement will eventually reach 1
> @@ -318,10 +323,6 @@ static void vhost_zerocopy_callback(struct ubuf_info *ubuf, bool success)
>  	 */
>  	if (cnt <= 2 || !(cnt % 16))
>  		vhost_poll_queue(&vq->poll);
> -	/* set len to mark this desc buffers done DMA */
> -	vq->heads[ubuf->desc].len = success ?
> -		VHOST_DMA_DONE_LEN : VHOST_DMA_FAILED_LEN;
> -	vhost_net_ubuf_put(ubufs);
>  }
>  
>  /* Expects to be always run from workqueue - which acts as
> -- 
> 1.7.1