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

[Jiri Pirko <jiri@resnulli.us>]

Wed, Jan 03, 2024 at 11:36:36PM CET, dw@davidwei.uk wrote:
>On 2024-01-02 03:13, Jiri Pirko wrote:
>> Thu, Dec 28, 2023 at 02:46:31AM CET, dw@davidwei.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
>> 
>> I don't see the rtnl_lock take in those functions.
>> 
>> 
>> Otherwise, this patch looks fine to me.
>
>For nsim_create(), rtnl_lock is taken in nsim_init_netdevsim(). For
>nsim_destroy(), rtnl_lock is taken directly in the function.
>
>What I mean here is, in the netdevsim device modification paths locks
>are taken in this order:
>
>devl_lock -> rtnl_lock
>
>nsim_dev_list_lock is taken outside (not nested) of these.
>
>In nsim_dev_peer_write() where two ports are linked, locks are taken in
>this order:
>
>nsim_dev_list_lock -> devl_lock -> rtnl_lock
>
>This will not cause deadlocks and ensures that two ports being linked
>are both valid.

Okay. Perhaps would be good to document this in a comment somewhere in
the code?


>
>> 
>> 
>>> concurrent calls to linking two netdevsims together.
>>>
>>> Signed-off-by: David Wei <dw@davidwei.uk>
>>> ---
>>> 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
>>>