[PATCH v2 net-next 4/4] net: reduce RTNL hold duration in unregister_netdevice_many_notify()

[Eric Dumazet <edumazet@google.com>]

Two synchronize_net() calls are currently done while holding RTNL.

This is source of RTNL contention in workloads adding and deleting
many network namespaces per second, because synchronize_rcu()
and synchronize_rcu_expedited() can use 10+ ms in some cases.

For cleanup_net() use, temporarily release RTNL
while calling synchronize_net().

This should be safe, because devices are no longer visible
to other threads after the call to unlist_netdevice(),
and the netns are in dismantle phase.

Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/core/dev.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 86fa9ae29d31a25dca8204c75fd39974ef84707d..dde5d40e8f6b1ad5309ebc6a6e163ddb96f66f73 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -11563,8 +11563,8 @@ void unregister_netdevice_many_notify(struct list_head *head,
 
 	rtnl_drop_if_cleanup_net();
 	flush_all_backlogs();
-	rtnl_acquire_if_cleanup_net();
 	synchronize_net();
+	rtnl_acquire_if_cleanup_net();
 
 	list_for_each_entry(dev, head, unreg_list) {
 		struct sk_buff *skb = NULL;
@@ -11624,7 +11624,9 @@ void unregister_netdevice_many_notify(struct list_head *head,
 #endif
 	}
 
+	rtnl_drop_if_cleanup_net();
 	synchronize_net();
+	rtnl_acquire_if_cleanup_net();
 
 	list_for_each_entry(dev, head, unreg_list) {
 		netdev_put(dev, &dev->dev_registered_tracker);
-- 
2.47.1.613.gc27f4b7a9f-goog