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

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Message-ID: <420b3c0a-6321-494b-9181-ff7dd4e1849c@machnikowski.net>
Date: Sat, 10 Feb 2024 22:43:43 +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 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

> +
> +	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);

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;