Subject: Re: [PATCH V2 5/6] vhost_net: poll vhost queue after marking DMA is done

From: Qin Chuanyu <[email-masked]>
Date: Wed, 12 Feb 2014 15:38:18 +0800

On 2013/8/30 12:29, 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]>
> ---
>   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 ff60c2a..d09c17c 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
>
with this change, vq would lose protection that provided by ubufs->kref.
if another thread is waiting at vhost_net_ubuf_put_and_wait called by
vhost_net_release, then after vhost_net_ubuf_put, vq would been free
by vhost_net_release soon, vhost_poll_queue(&vq->poll) may cause NULL
pointer Exception.

another question is that vhost_zerocopy_callback is called by kfree_skb,
it may called in different thread context.
vhost_poll_queue is called decided by ubufs->kref.refcount, this may 
cause there isn't any thread call  vhost_poll_queue, but at least one is 
needed. and this cause network break.
We could repeat it by using 8 netperf thread in guest to xmit tcp to its 
host.

I think if using atomic_read to decide while do vhost_poll_queue or not,
at least a spink_lock is needed.