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

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Message-ID: <4f9a912b-db9a-4e66-8dd1-b0a719e2cb9f@davidwei.uk>
Date: Mon, 12 Feb 2024 10:25:47 -0800
From: David Wei <dw@...idwei.uk>
To: Maciek Machnikowski <maciek@...hnikowski.net>,
 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 2024-02-12 10:10, Maciek Machnikowski wrote:
> 
> 
> 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().

Oh, I see now, thank you.

> 
>>
>>>
>>> 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;