net: avoid updating qdisc_xmit_lock_key in netdev_update_lockdep_key()

syzbot reported some bogus lockdep warnings, for example bad unlock
balance in sch_direct_xmit(). They are due to a race condition between
slow path and fast path, that is qdisc_xmit_lock_key gets re-registered
in netdev_update_lockdep_key() on slow path, while we could still
acquire the queue->_xmit_lock on fast path in this small window:

CPU A						CPU B
						__netif_tx_lock();
lockdep_unregister_key(qdisc_xmit_lock_key);
						__netif_tx_unlock();
lockdep_register_key(qdisc_xmit_lock_key);

In fact, unlike the addr_list_lock which has to be reordered when
the master/slave device relationship changes, queue->_xmit_lock is
only acquired on fast path and only when NETIF_F_LLTX is not set,
so there is likely no nested locking for it.

Therefore, we can just get rid of re-registration of
qdisc_xmit_lock_key.

Reported-by: syzbot+4ec99438ed7450da6272@syzkaller.appspotmail.com
Fixes: ab92d68f ("net: core: add generic lockdep keys")
Cc: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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);