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

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Message-ID: <1377881073.2065.10.camel@bwh-desktop.uk.level5networks.com>
Date:	Fri, 30 Aug 2013 17:44:33 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Jason Wang <jasowang@...hat.com>
CC:	<mst@...hat.com>, <kvm@...r.kernel.org>,
	<virtualization@...ts.linux-foundation.org>,
	<netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 5/6] vhost_net: poll vhost queue after marking DMA
 is done

On Fri, 2013-08-30 at 12:29 +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.

Does this bug only exist in net-next or is it older?  Should the fix go
to net and stable branches?

Ben.

> Signed-off-by: Jason Wang <jasowang@...hat.com>
> ---
>  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

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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