net: avoid quadratic behavior in netdev_wait_allrefs_any()

If the list of devices has N elements, netdev_wait_allrefs_any()
is called N times, and linkwatch_forget_dev() is called N*(N-1)/2 times.

Fix this by calling linkwatch_forget_dev() only once per device.

Fixes: faab39f ("net: allow out-of-order netdev unregistration")
Signed-off-by: Eric Dumazet <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: edumazet

net/core/dev.c

@@ -9828,9 +9828,6 @@ static struct net_device *netdev_wait_allrefs_any(struct list_head *list)
 	struct net_device *dev;
 	int wait = 0;
 
-	list_for_each_entry(dev, list, todo_list)
-		linkwatch_forget_dev(dev);
-
 	rebroadcast_time = warning_time = jiffies;
 
 	list_for_each_entry(dev, list, todo_list)
@@ -9951,6 +9948,7 @@ void netdev_run_todo(void)
 		}
 
 		dev->reg_state = NETREG_UNREGISTERED;
+		linkwatch_forget_dev(dev);
 	}
 
 	while (!list_empty(&list)) {