netdev - Re: virtio_net: suspicious RCU usage with xdp

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Date:   Thu, 25 Apr 2019 10:56:03 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     David Ahern <dsahern@...il.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     makita.toshiaki@....ntt.co.jp,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Toke Høiland-Jørgensen <toke@...e.dk>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: virtio_net: suspicious RCU usage with xdp


On 2019/4/25 上午1:40, David Ahern wrote:
> On 4/24/19 11:37 AM, Michael S. Tsirkin wrote:
>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>> index 9010938e2d71..ccc1bdd1bb1f 100644
>> --- a/drivers/net/virtio_net.c
>> +++ b/drivers/net/virtio_net.c
>> @@ -495,8 +495,8 @@ static struct send_queue *virtnet_xdp_sq(struct virtnet_info *vi)
>>   	return &vi->sq[qp];
>>   }
>>   
>> -static int virtnet_xdp_xmit(struct net_device *dev,
>> -			    int n, struct xdp_frame **frames, u32 flags)
>> +static int __virtnet_xdp_xmit(struct net_device *dev,
>> +			      int n, struct xdp_frame **frames, u32 flags)
>>   {
>>   	struct virtnet_info *vi = netdev_priv(dev);
>>   	struct receive_queue *rq = vi->rq;
>> @@ -569,6 +569,17 @@ static int virtnet_xdp_xmit(struct net_device *dev,
>>   	return ret;
>>   }
>>   
>> +static int virtnet_xdp_xmit(struct net_device *dev,
>> +			    int n, struct xdp_frame **frames, u32 flags)
>> +{
>> +	int r;
>> +
>> +	rcu_read_lock_bh();
>> +	r = __virtnet_xdp_xmit(dev, n, frames, flags);
>> +	rcu_read_unlock_bh();
>> +	return r;
>> +}
>> +
>>   static unsigned int virtnet_get_headroom(struct virtnet_info *vi)
>>   {
>>   	return vi->xdp_queue_pairs ? VIRTIO_XDP_HEADROOM : 0;
>> @@ -714,7 +725,7 @@ static struct sk_buff *receive_small(struct net_device *dev,
> receive_small takes the rcu lock at the beginning.


Yes, so the code call __virtnet_xdp_xmit() that won't try to take rcu lock.

Thanks


>
>
>>   			xdpf = convert_to_xdp_frame(&xdp);
>>   			if (unlikely(!xdpf))
>>   				goto err_xdp;
>> -			err = virtnet_xdp_xmit(dev, 1, &xdpf, 0);
>> +			err = __virtnet_xdp_xmit(dev, 1, &xdpf, 0);
>>   			if (unlikely(err < 0)) {
>>   				trace_xdp_exception(vi->dev, xdp_prog, act);
>>   				goto err_xdp;
>> @@ -887,7 +898,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
> same here.
>>   			xdpf = convert_to_xdp_frame(&xdp);
>>   			if (unlikely(!xdpf))
>>   				goto err_xdp;
>> -			err = virtnet_xdp_xmit(dev, 1, &xdpf, 0);
>> +			err = __virtnet_xdp_xmit(dev, 1, &xdpf, 0);
>>   			if (unlikely(err < 0)) {
>>   				trace_xdp_exception(vi->dev, xdp_prog, act);
>>   				if (unlikely(xdp_page != page))
>>
>>
> bq_xmit_all is invoking the xdp_xmit callback without taking the rcu
> lock, but from the git history does not appear to have any recent changes.