net: ensure net_todo_list is processed quickly

In [1], Will raised a potential issue that the cfg80211 code,
which does (from a locking perspective)

rtnl_lock()
wiphy_lock()
rtnl_unlock()

might be suspectible to ABBA deadlocks, because rtnl_unlock()
calls netdev_run_todo(), which might end up calling rtnl_lock()
again, which could then deadlock (see the comment in the code
added here for the scenario).

Some back and forth and thinking ensued, but clearly this can't
happen if the net_todo_list is empty at the rtnl_unlock() here.
Clearly, the code here cannot actually put an entry on it, and
all other users of rtnl_unlock() will empty it since that will
always go through netdev_run_todo(), emptying the list.

So the only other way to get there would be to add to the list
and then unlock the RTNL without going through rtnl_unlock(),
which is only possible through __rtnl_unlock(). However, this
isn't exported and not used in many places, and none of them
seem to be able to unregister before using it.

Therefore, add a WARN_ON() in the code to ensure this invariant
won't be broken, so that the cfg80211 (or any similar) code
stays safe.

[1] https://lore.kernel.org/r/[e-mail]

Signed-off-by: Johannes Berg <[e-mail]>
Link: https://lore.kernel.org/r/20220404113847.0ee02e4a70da.Ic73d206e217db20fd22dcec14fe5442ca732804b@changeid
Signed-off-by: Jakub Kicinski <[e-mail]>

include/linux/netdevice.h

···
 extern int		netdev_budget;
 extern unsigned int	netdev_budget_usecs;
 
-/* Called by rtnetlink.c:rtnl_unlock() */
+/* Used by rtnetlink.c:__rtnl_unlock()/rtnl_unlock() */
+extern struct list_head net_todo_list;
 void netdev_run_todo(void);
 
 static inline void __dev_put(struct net_device *dev)

net/core/dev.c

···
 }
 
 /* Delayed registration/unregisteration */
-static LIST_HEAD(net_todo_list);
+LIST_HEAD(net_todo_list);
 DECLARE_WAIT_QUEUE_HEAD(netdev_unregistering_wq);
 
 static void net_set_todo(struct net_device *dev)

net/core/rtnetlink.c

···
 
 	defer_kfree_skb_list = NULL;
 
+	/* Ensure that we didn't actually add any TODO item when __rtnl_unlock()
+	 * is used. In some places, e.g. in cfg80211, we have code that will do
+	 * something like
+	 *   rtnl_lock()
+	 *   wiphy_lock()
+	 *   ...
+	 *   rtnl_unlock()
+	 *
+	 * and because netdev_run_todo() acquires the RTNL for items on the list
+	 * we could cause a situation such as this:
+	 * Thread 1			Thread 2
+	 *				  rtnl_lock()
+	 *				  unregister_netdevice()
+	 *				  __rtnl_unlock()
+	 * rtnl_lock()
+	 * wiphy_lock()
+	 * rtnl_unlock()
+	 *   netdev_run_todo()
+	 *     __rtnl_unlock()
+	 *
+	 *     // list not empty now
+	 *     // because of thread 2
+	 *				  rtnl_lock()
+	 *     while (!list_empty(...))
+	 *       rtnl_lock()
+	 *				  wiphy_lock()
+	 * **** DEADLOCK ****
+	 *
+	 * However, usage of __rtnl_unlock() is rare, and so we can ensure that
+	 * it's not used in cases where something is added to do the list.
+	 */
+	WARN_ON(!list_empty(&net_todo_list));
+
 	mutex_unlock(&rtnl_mutex);
 
 	while (head) {