netdev - Re: [PATCH net-next v9 2/3] netdevsim: forward skbs from one connected port to another

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Message-ID: <49aece93-d9a8-4cd8-be79-2160f375e5af@machnikowski.net>
Date: Mon, 12 Feb 2024 19:10:16 +0100
From: Maciek Machnikowski <maciek@...hnikowski.net>
To: David Wei <dw@...idwei.uk>, Jakub Kicinski <kuba@...nel.org>,
 Jiri Pirko <jiri@...nulli.us>, Sabrina Dubroca <sd@...asysnail.net>,
 netdev@...r.kernel.org
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net-next v9 2/3] netdevsim: forward skbs from one
 connected port to another



On 12/02/2024 18:32, David Wei wrote:
> On 2024-02-10 13:43, Maciek Machnikowski wrote:
>>
>>
>> On 10/02/2024 01:32, David Wei wrote:
>>> Forward skbs sent from one netdevsim port to its connected netdevsim
>>> port using dev_forward_skb, in a spirit similar to veth.
>>>
>>> Add a tx_dropped variable to struct netdevsim, tracking the number of
>>> skbs that could not be forwarded using dev_forward_skb().
>>>
>>> The xmit() function accessing the peer ptr is protected by an RCU read
>>> critical section. The rcu_read_lock() is functionally redundant as since
>>> v5.0 all softirqs are implicitly RCU read critical sections; but it is
>>> useful for human readers.
>>>
>>> If another CPU is concurrently in nsim_destroy(), then it will first set
>>> the peer ptr to NULL. This does not affect any existing readers that
>>> dereferenced a non-NULL peer. Then, in unregister_netdevice(), there is
>>> a synchronize_rcu() before the netdev is actually unregistered and
>>> freed. This ensures that any readers i.e. xmit() that got a non-NULL
>>> peer will complete before the netdev is freed.
>>>
>>> Any readers after the RCU_INIT_POINTER() but before synchronize_rcu()
>>> will dereference NULL, making it safe.
>>>
>>> The codepath to nsim_destroy() and nsim_create() takes both the newly
>>> added nsim_dev_list_lock and rtnl_lock. This makes it safe with
>>> concurrent calls to linking two netdevsims together.
>>>
>>> Signed-off-by: David Wei <dw@...idwei.uk>
>>> ---
>>>  drivers/net/netdevsim/netdev.c    | 28 +++++++++++++++++++++++-----
>>>  drivers/net/netdevsim/netdevsim.h |  1 +
>>>  2 files changed, 24 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/drivers/net/netdevsim/netdev.c b/drivers/net/netdevsim/netdev.c
>>> index 9063f4f2971b..13d3e1536451 100644
>>> --- a/drivers/net/netdevsim/netdev.c
>>> +++ b/drivers/net/netdevsim/netdev.c
>>> @@ -29,19 +29,37 @@
>>>  static netdev_tx_t nsim_start_xmit(struct sk_buff *skb, struct net_device *dev)
>>>  {
>>>  	struct netdevsim *ns = netdev_priv(dev);
>>> +	unsigned int len = skb->len;
>>> +	struct netdevsim *peer_ns;
>>> +	int ret = NETDEV_TX_OK;
>>>  
>>>  	if (!nsim_ipsec_tx(ns, skb))
>>>  		goto out;
>>>  
>>> +	rcu_read_lock();
>>> +	peer_ns = rcu_dereference(ns->peer);
>>> +	if (!peer_ns)
>>> +		goto out_stats;
>> Change ret to NET_XMIT_DROP to correctly count packets as dropped
> 
> Linking and forwarding between two netdevsims is optional, so I don't
> want to always return NET_XMIT_DROP.
> OK - now I see that previously we calculated all packets as TX_OK and
transmitted, so this is OK.

>>
>>> +
>>> +	skb_tx_timestamp(skb);
>>> +	if (unlikely(dev_forward_skb(peer_ns->netdev, skb) == NET_RX_DROP))
>>> +		ret = NET_XMIT_DROP;
>>> +
>>> +out_stats:
>>> +	rcu_read_unlock();
>>>  	u64_stats_update_begin(&ns->syncp);
>>> -	ns->tx_packets++;
>>> -	ns->tx_bytes += skb->len;
>>> +	if (ret == NET_XMIT_DROP) {
>>> +		ns->tx_dropped++;
>> add dev_kfree_skb(skb);
> 
> dev_forward_skb() frees the skb if dropped already.
But the dev_forward_skb() won't be called if there is no peer_ns. In
this case we still need to call dev_kfree_skb().

> 
>>
>> Thanks,
>> Maciek
>>
>>> +	} else {
>>> +		ns->tx_packets++;
>>> +		ns->tx_bytes += len;
>>> +	}
>>>  	u64_stats_update_end(&ns->syncp);
>>> +	return ret;
>>>  
>>>  out:
>>>  	dev_kfree_skb(skb);
>>> -
>>> -	return NETDEV_TX_OK;
>>> +	return ret;
>>>  }
>>>  
>>>  static void nsim_set_rx_mode(struct net_device *dev)
>>> @@ -70,6 +88,7 @@ nsim_get_stats64(struct net_device *dev, struct rtnl_link_stats64 *stats)
>>>  		start = u64_stats_fetch_begin(&ns->syncp);
>>>  		stats->tx_bytes = ns->tx_bytes;
>>>  		stats->tx_packets = ns->tx_packets;
>>> +		stats->tx_dropped = ns->tx_dropped;
>>>  	} while (u64_stats_fetch_retry(&ns->syncp, start));
>>>  }
>>>  
>>> @@ -302,7 +321,6 @@ static void nsim_setup(struct net_device *dev)
>>>  	eth_hw_addr_random(dev);
>>>  
>>>  	dev->tx_queue_len = 0;
>>> -	dev->flags |= IFF_NOARP;
>>>  	dev->flags &= ~IFF_MULTICAST;
>>>  	dev->priv_flags |= IFF_LIVE_ADDR_CHANGE |
>>>  			   IFF_NO_QUEUE;
>>> diff --git a/drivers/net/netdevsim/netdevsim.h b/drivers/net/netdevsim/netdevsim.h
>>> index c8b45b0d955e..553c4b9b4f63 100644
>>> --- a/drivers/net/netdevsim/netdevsim.h
>>> +++ b/drivers/net/netdevsim/netdevsim.h
>>> @@ -98,6 +98,7 @@ struct netdevsim {
>>>  
>>>  	u64 tx_packets;
>>>  	u64 tx_bytes;
>>> +	u64 tx_dropped;
>>>  	struct u64_stats_sync syncp;
>>>  
>>>  	struct nsim_bus_dev *nsim_bus_dev;