linux-kernel - Re: [PATCH net] virtio-net: don't free skb immediately on kick failure

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Date:	Mon, 24 Mar 2014 08:27:24 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	rusty@...tcorp.com.au, virtualization@...ts.linux-foundation.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Heinz Graalfs <graalfs@...ux.vnet.ibm.com>
Subject: Re: [PATCH net] virtio-net: don't free skb immediately on kick
 failure

On Mon, Mar 24, 2014 at 01:31:57PM +0800, Jason Wang wrote:
> On 03/23/2014 07:27 PM, Michael S. Tsirkin wrote:
> > On Fri, Mar 21, 2014 at 04:30:01PM +0800, Jason Wang wrote:
> >> We free the skb immediately on kick failure during xmit without detaching it
> >> from the virtqueue. This may lead double free for the skb during
> >> free_unused_bufs(). This patch fixes this by not freeing it on kick failure and
> >> let it to be freed through free_unused_bufs().
> >>
> >> Fixes 67975901183799af8e93ec60e322f9e2a1940b9b
> >> ("virtio_net: verify if virtqueue_kick() succeeded").
> > It's not even obvious we want to increment fifo errors
> > counter.
> > So basically we want to ignore these errors.
> > In that case how about just reverting this chunk
> > of the patch?
> 
> Sure, I think it's safe to do this.
> > Seems cleaner than adding more code.
> >
> > The chunk filling in RX also seems inappropriate:
> > it will cause RX work to be requeued which isn't what
> > we wanted.
> 
> True, we can also ignore the error here.

So let's do that - just revert most of that commit
except the bit that breaks out of look when we submit commands.

> >
> > Also, what happens with virtnet_send_command?
> 
> Looks ok, if vq is broken it will return error to the caller or jump out
> of the busy loop.
> > Looks like that patch is completely wrong.
> >
> >> Cc: Rusty Russell <rusty@...tcorp.com.au>
> >> Cc: Michael S. Tsirkin <mst@...hat.com>
> >> Cc: Heinz Graalfs <graalfs@...ux.vnet.ibm.com>
> >> Signed-off-by: Jason Wang <jasowang@...hat.com>
> >> ---
> >>  drivers/net/virtio_net.c | 6 ++++--
> >>  1 file changed, 4 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> >> index 5632a99..d833d38 100644
> >> --- a/drivers/net/virtio_net.c
> >> +++ b/drivers/net/virtio_net.c
> >> @@ -882,8 +882,10 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
> >>  		if (net_ratelimit())
> >>  			dev_warn(&dev->dev,
> >>  				 "Unexpected TXQ (%d) queue failure: %d\n", qnum, err);
> >> -		dev->stats.tx_dropped++;
> >> -		kfree_skb(skb);
> >> +		if (err) {
> >> +			dev->stats.tx_dropped++;
> >> +			kfree_skb(skb);
> >> +		}
> >>  		return NETDEV_TX_OK;
> >>  	}
> >>  
> >> -- 
> >> 1.8.3.2
> > --
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