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Message-ID: <7694087c-6867-45ec-ba72-cc0c12933903@davidwei.uk>
Date: Thu, 21 Dec 2023 16:58:29 -0800
From: David Wei <dw@...idwei.uk>
To: Jiri Pirko <jiri@...nulli.us>
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
On 2023-12-20 01:04, Jiri Pirko wrote:
> 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()
I see, it's a type of spinlock. I will fix this.
>
>
>> + }
>> +
>> + 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
>>
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