On Sat, Jan 11, 2020 at 1:53 PM Cong Wang [off-list ref] wrote:

The details you provide here are really helpful for me to understand
the reasons behind your changes. Let me think about this and see how
I could address both problems. This appears to be harder than I originally
thought.

Do you think the following patch will make everyone happy?

diff --git a/net/core/dev.c b/net/core/dev.c
index 0ad39c87b7fd..7e885d069707 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c

@@ -9177,22 +9177,10 @@ static void

netdev_unregister_lockdep_key(struct net_device *dev)

 void netdev_update_lockdep_key(struct net_device *dev)
 {
-       struct netdev_queue *queue;
-       int i;
-
-       lockdep_unregister_key(&dev->qdisc_xmit_lock_key);
        lockdep_unregister_key(&dev->addr_list_lock_key);
-
-       lockdep_register_key(&dev->qdisc_xmit_lock_key);
        lockdep_register_key(&dev->addr_list_lock_key);

        lockdep_set_class(&dev->addr_list_lock, &dev->addr_list_lock_key);
-       for (i = 0; i < dev->num_tx_queues; i++) {
-               queue = netdev_get_tx_queue(dev, i);
-
-               lockdep_set_class(&queue->_xmit_lock,
-                                 &dev->qdisc_xmit_lock_key);
-       }
 }
 EXPORT_SYMBOL(netdev_update_lockdep_key);

I think as long as we don't take _xmit_lock nestedly, it is fine. And
most (or all?) of the software netdev's are already lockless, so I can't
think of any case we take more than one _xmit_lock on TX path.

I tested it with the syzbot reproducer and your set master/nomaster
commands, I don't get any lockdep splat.

What do you think?

Thanks!