Subject: Re: [PATCH net-next v5 3/5] netdevsim: forward skbs from one connected port to another

From: Jiri Pirko <[email-masked]>
Date: Tue, 2 Jan 2024 12:13:39 +0100

Thu, Dec 28, 2023 at 02:46:31AM CET, [email-masked] 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

I don't see the rtnl_lock take in those functions.

Otherwise, this patch looks fine to me.

>concurrent calls to linking two netdevsims together.
>
>Signed-off-by: David Wei <[email-masked]>
>---
> drivers/net/netdevsim/netdev.c    | 21 ++++++++++++++++++---
> drivers/net/netdevsim/netdevsim.h |  1 +
> 2 files changed, 19 insertions(+), 3 deletions(-)
>
>diff --git a/drivers/net/netdevsim/netdev.c b/drivers/net/netdevsim/netdev.c
>index 434322f6a565..0009d0f1243f 100644
>--- a/drivers/net/netdevsim/netdev.c
>+++ b/drivers/net/netdevsim/netdev.c
>@@ -29,19 +29,34 @@
> 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;
> 
>+	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;
>+
>+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++;
> 	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 +85,7 @@ 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 +318,6 @@ 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
>