Re: for_each_netdev_rcu() protected by RTNL and CONFIG_PROVE_RCU_LIST

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Breno Leitao <leitao@debian.org>
Date: Thu, 6 Feb 2025 07:51:55 -0800
> Hello,
> 
> We're seeing CONFIG_PROVE_RCU_LIST warnings when for_each_netdev_rcu()
> is called with RTNL held. While RTNL provides sufficient locking, the
> RCU list checker isn't aware of this relationship, leading to false
> positives like:
> 
> 	WARNING: suspicious RCU usage
> 	net/core/dev.c:1143 RCU-list traversed in non-reader section!!
> 
> The initial discussion popped up in:
> 
> 	https://lore.kernel.org/all/20250205-flying-coucal-of-influence-0dcbc3@leitao/
> 
> I've attempted a solution by modifying for_each_netdev_rcu():
> 
> 	diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> 	index 2a59034a5fa2f..59b18b58fa927 100644
> 	--- a/include/linux/netdevice.h
> 	+++ b/include/linux/netdevice.h
> 	@@ -3210,13 +3210,14 @@ netdev_notifier_info_to_extack(const struct netdev_notifier_info *info)
> 	int call_netdevice_notifiers(unsigned long val, struct net_device *dev);
> 	int call_netdevice_notifiers_info(unsigned long val,
> 					struct netdev_notifier_info *info);
> 	+bool lockdep_rtnl_net_is_held(struct net *net);
> 
> 	#define for_each_netdev(net, d)		\
> 			list_for_each_entry(d, &(net)->dev_base_head, dev_list)
> 	#define for_each_netdev_reverse(net, d)	\
> 			list_for_each_entry_reverse(d, &(net)->dev_base_head, dev_list)
> 	#define for_each_netdev_rcu(net, d)		\
> 	-		list_for_each_entry_rcu(d, &(net)->dev_base_head, dev_list)
> 	+		list_for_each_entry_rcu(d, &(net)->dev_base_head, dev_list, lockdep_rtnl_net_is_held(net))
> 	#define for_each_netdev_safe(net, d, n)	\
> 			list_for_each_entry_safe(d, n, &(net)->dev_base_head, dev_list)
> 	#define for_each_netdev_continue(net, d)		\
> 
> However, I have concerns about using lockdep_rtnl_net_is_held() since it
> has a dependency on CONFIG_DEBUG_NET_SMALL_RTNL.
> 
> Are there better approaches to silence these warnings when RTNL is held?
> Any suggestions would be appreciated.

We can't use lockdep_rtnl_net_is_held() there yet because most users are
not converted to per-netns RTNL, so it will complain loudly.