From: ebiederm@xmission.com (Eric W. Biederman)
Date: Mon, 16 Sep 2013 20:49:31 -0700

 /* Delayed registration/unregisteration */
 static LIST_HEAD(net_todo_list);
+static atomic_t netdev_unregistering = ATOMIC_INIT(0);
+static DECLARE_WAIT_QUEUE_HEAD(netdev_unregistering_wait);

I think you don't need this atomic.

Something like this should work and seems much simpler:

1) Still use the netdev_unregistering_wait queue, that's fine.

2) In netdev_run_todo(), unconditionally wake it up at the end of
   the while() loop.

3) In default_device_exit_batch() use the same retry logic but
   your tests are on list_empty(&net_todo_list() rather than the
   new atomic count.

I think with this simplification I'm fine to apply this patch after
it has been tested properly.

Long term I'd like to see a per-namespace todo list, as seems to
have been suggested and discussed already.

Thanks.