[PATCH v1 net-next 2/5] net: Add ->pre_exit_batch() to struct pernet_operations.

[Kuniyuki Iwashima <kuniyu@google.com>]

While unregistering a netdev, we must find and clean up all
resources holding refcounts of the device.

This operation is usually triggered for each component via
NETDEV_UNREGISTER and typically requires a full scan of hash
tables, etc.

For example, rt_flush_dev() and rt6_uncached_list_flush_dev()
are called for each netdev and iterate over the global per-cpu
lists to find uncached routes tied to the device, resulting
in O(N_dev x (N_cpu + N_route)) loops during netns dismantle.

Moreover, rt_flush_dev() and rt6_uncached_list_flush_dev()
do not even require RTNL.

To batch such cleanup across dying netns without holding RTNL
before ops_exit_rtnl_list(), let's add a new ->pre_exit_batch()
hook to pernet_operations.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 include/net/net_namespace.h | 1 +
 net/core/net_namespace.c    | 3 +++
 2 files changed, 4 insertions(+)

diff --git a/include/net/net_namespace.h b/include/net/net_namespace.h
index b4247e767d13..58b2601bb869 100644
--- a/include/net/net_namespace.h
+++ b/include/net/net_namespace.h
@@ -488,6 +488,7 @@ struct pernet_operations {
 	 */
 	int (*init)(struct net *net);
 	void (*pre_exit)(struct net *net);
+	void (*pre_exit_batch)(struct list_head *net_exit_list);
 	void (*exit)(struct net *net);
 	void (*exit_batch)(struct list_head *net_exit_list);
 	/* Following method is called with RTNL held. */
diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
index 0e13de0cd36b..476fbf913bad 100644
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c
@@ -160,6 +160,9 @@ static void ops_pre_exit_list(const struct pernet_operations *ops,
 		list_for_each_entry(net, net_exit_list, exit_list)
 			ops->pre_exit(net);
 	}
+
+	if (ops->pre_exit_batch)
+		ops->pre_exit_batch(net_exit_list);
 }
 
 static void ops_exit_rtnl_list(const struct list_head *ops_list,
-- 
2.56.0.rc1.315.gc6ed9934b7-goog