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

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Date:	Mon, 24 Mar 2014 13:31:57 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	"Michael S. Tsirkin" <mst@...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 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.
>
> 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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