netdev - Re: [PATCH net-next v4 3/5] netdevsim: forward skbs from one connected port to another

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Date: Wed, 20 Dec 2023 10:04:14 +0100
From: Jiri Pirko <jiri@...nulli.us>
To: David Wei <dw@...idwei.uk>
Cc: Jakub Kicinski <kuba@...nel.org>, Sabrina Dubroca <sd@...asysnail.net>,
	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net-next v4 3/5] netdevsim: forward skbs from one
 connected port to another

Wed, Dec 20, 2023 at 02:47:45AM CET, dw@...idwei.uk 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    | 25 ++++++++++++++++++++++---
> drivers/net/netdevsim/netdevsim.h |  1 +
> 2 files changed, 23 insertions(+), 3 deletions(-)
>
>diff --git a/drivers/net/netdevsim/netdev.c b/drivers/net/netdevsim/netdev.c
>index 434322f6a565..00ab3098eb9f 100644
>--- a/drivers/net/netdevsim/netdev.c
>+++ b/drivers/net/netdevsim/netdev.c
>@@ -29,6 +29,8 @@
> static netdev_tx_t nsim_start_xmit(struct sk_buff *skb, struct net_device *dev)
> {
> 	struct netdevsim *ns = netdev_priv(dev);
>+	struct netdevsim *peer_ns;
>+	int ret = NETDEV_TX_OK;
> 
> 	if (!nsim_ipsec_tx(ns, skb))
> 		goto out;
>@@ -36,12 +38,29 @@ static netdev_tx_t nsim_start_xmit(struct sk_buff *skb, struct net_device *dev)
> 	u64_stats_update_begin(&ns->syncp);
> 	ns->tx_packets++;
> 	ns->tx_bytes += skb->len;
>+
>+	rcu_read_lock();
>+	peer_ns = rcu_dereference(ns->peer);
>+	if (!peer_ns)
>+		goto out_stats;
>+
>+	skb_tx_timestamp(skb);
>+	if (unlikely(dev_forward_skb(peer_ns->netdev, skb) == NET_RX_DROP)) {
>+		ret = NET_XMIT_DROP;
>+		ns->tx_dropped++;

Idk, does not look fine to me to be in u64_stats_update section while
calling dev_forward_skb()


>+	}
>+
>+	rcu_read_unlock();
> 	u64_stats_update_end(&ns->syncp);
> 
>+	return ret;
>+
>+out_stats:
>+	rcu_read_unlock();
>+	u64_stats_update_end(&ns->syncp);
> out:
> 	dev_kfree_skb(skb);
>-
>-	return NETDEV_TX_OK;
>+	return ret;
> }
> 
> static void nsim_set_rx_mode(struct net_device *dev)
>@@ -70,6 +89,7 @@ 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 +322,6 @@ 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 24fc3fbda791..083b1ee7a1a2 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;
>-- 
>2.39.3
>