net: Add ->exit_rtnl() hook to struct pernet_operations.

struct pernet_operations provides two batching hooks; ->exit_batch()
and ->exit_batch_rtnl().

The batching variant is beneficial if ->exit() meets any of the
following conditions:

  1) ->exit() repeatedly acquires a global lock for each netns

  2) ->exit() has a time-consuming operation that can be factored
     out (e.g. synchronize_rcu(), smp_mb(), etc)

  3) ->exit() does not need to repeat the same iterations for each
     netns (e.g. inet_twsk_purge())

Currently, none of the ->exit_batch_rtnl() functions satisfy any of
the above conditions because RTNL is factored out and held by the
caller and all of these functions iterate over the dying netns list.

Also, we want to hold per-netns RTNL there but avoid spreading
__rtnl_net_lock() across multiple locations.

Let's add ->exit_rtnl() hook and run it under __rtnl_net_lock().

The following patches will convert all ->exit_batch_rtnl() users
to ->exit_rtnl().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/20250411205258.63164-4-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/net/net_namespace.h b/include/net/net_namespace.h
index bd57d8fb54f1..b071e6eed9d5 100644
--- a/include/net/net_namespace.h
+++ b/include/net/net_namespace.h
@@ -475,6 +475,8 @@ struct pernet_operations {
 	void (*exit)(struct net *net);
 	void (*exit_batch)(struct list_head *net_exit_list);
 	/* Following method is called with RTNL held. */
+	void (*exit_rtnl)(struct net *net,
+			  struct list_head *dev_kill_list);
 	void (*exit_batch_rtnl)(struct list_head *net_exit_list,
 				struct list_head *dev_kill_list);
 	unsigned int * const id;