net/sched: fix false lockdep warning on qdisc root lock

Xiumei and Christoph reported the following lockdep splat, complaining of
the qdisc root lock being taken twice:

============================================
WARNING: possible recursive locking detected
6.7.0-rc3+ #598 Not tainted
--------------------------------------------
swapper/2/0 is trying to acquire lock:
ffff888177190110 (&sch->q.lock){+.-.}-{2:2}, at: __dev_queue_xmit+0x1560/0x2e70

but task is already holding lock:
ffff88811995a110 (&sch->q.lock){+.-.}-{2:2}, at: __dev_queue_xmit+0x1560/0x2e70

other info that might help us debug this:
Possible unsafe locking scenario:

CPU0
----
lock(&sch->q.lock);
lock(&sch->q.lock);

*** DEADLOCK ***

May be due to missing lock nesting notation

5 locks held by swapper/2/0:
#0: ffff888135a09d98 ((&in_dev->mr_ifc_timer)){+.-.}-{0:0}, at: call_timer_fn+0x11a/0x510
#1: ffffffffaaee5260 (rcu_read_lock){....}-{1:2}, at: ip_finish_output2+0x2c0/0x1ed0
#2: ffffffffaaee5200 (rcu_read_lock_bh){....}-{1:2}, at: __dev_queue_xmit+0x209/0x2e70
#3: ffff88811995a110 (&sch->q.lock){+.-.}-{2:2}, at: __dev_queue_xmit+0x1560/0x2e70
#4: ffffffffaaee5200 (rcu_read_lock_bh){....}-{1:2}, at: __dev_queue_xmit+0x209/0x2e70

stack backtrace:
CPU: 2 PID: 0 Comm: swapper/2 Not tainted 6.7.0-rc3+ #598
Hardware name: Red Hat KVM, BIOS 1.13.0-2.module+el8.3.0+7353+9de0a3cc 04/01/2014
Call Trace:
<IRQ>
dump_stack_lvl+0x4a/0x80
__lock_acquire+0xfdd/0x3150
lock_acquire+0x1ca/0x540
_raw_spin_lock+0x34/0x80
__dev_queue_xmit+0x1560/0x2e70
tcf_mirred_act+0x82e/0x1260 [act_mirred]
tcf_action_exec+0x161/0x480
tcf_classify+0x689/0x1170
prio_enqueue+0x316/0x660 [sch_prio]
dev_qdisc_enqueue+0x46/0x220
__dev_queue_xmit+0x1615/0x2e70
ip_finish_output2+0x1218/0x1ed0
__ip_finish_output+0x8b3/0x1350
ip_output+0x163/0x4e0
igmp_ifc_timer_expire+0x44b/0x930
call_timer_fn+0x1a2/0x510
run_timer_softirq+0x54d/0x11a0
__do_softirq+0x1b3/0x88f
irq_exit_rcu+0x18f/0x1e0
sysvec_apic_timer_interrupt+0x6f/0x90
</IRQ>

This happens when TC does a mirred egress redirect from the root qdisc of
device A to the root qdisc of device B. As long as these two locks aren't
protecting the same qdisc, they can be acquired in chain: add a per-qdisc
lockdep key to silence false warnings.
This dynamic key should safely replace the static key we have in sch_htb:
it was added to allow enqueueing to the device "direct qdisc" while still
holding the qdisc root lock.

v2: don't use static keys anymore in HTB direct qdiscs (thanks Eric Dumazet)

CC: Maxim Mikityanskiy <[e-mail]>
CC: Xiumei Mu <[e-mail]>
Reported-by: Christoph Paasch <[e-mail]>
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/451
Signed-off-by: Davide Caratti <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

include/net/sch_generic.h

···
 
 	struct rcu_head		rcu;
 	netdevice_tracker	dev_tracker;
+	struct lock_class_key	root_lock_key;
 	/* private data */
 	long privdata[] ____cacheline_aligned;
 };

net/sched/sch_generic.c

···
 	__skb_queue_head_init(&sch->gso_skb);
 	__skb_queue_head_init(&sch->skb_bad_txq);
 	gnet_stats_basic_sync_init(&sch->bstats);
+	lockdep_register_key(&sch->root_lock_key);
 	spin_lock_init(&sch->q.lock);
+	lockdep_set_class(&sch->q.lock, &sch->root_lock_key);
 
 	if (ops->static_flags & TCQ_F_CPUSTATS) {
 		sch->cpu_bstats =
···
 	if (ops->destroy)
 		ops->destroy(qdisc);
 
+	lockdep_unregister_key(&qdisc->root_lock_key);
 	module_put(ops->owner);
 	netdev_put(dev, &qdisc->dev_tracker);
 

net/sched/sch_htb.c

···
 	rcu_read_unlock();
 }
 
-static void htb_set_lockdep_class_child(struct Qdisc *q)
-{
-	static struct lock_class_key child_key;
-
-	lockdep_set_class(qdisc_lock(q), &child_key);
-}
-
 static int htb_offload(struct net_device *dev, struct tc_htb_qopt_offload *opt)
 {
 	return dev->netdev_ops->ndo_setup_tc(dev, TC_SETUP_QDISC_HTB, opt);
···
 			return -ENOMEM;
 		}
 
-		htb_set_lockdep_class_child(qdisc);
 		q->direct_qdiscs[ntx] = qdisc;
 		qdisc->flags |= TCQ_F_ONETXQUEUE | TCQ_F_NOPARENT;
 	}
···
 	}
 
 	if (q->offload) {
-		htb_set_lockdep_class_child(new);
 		/* One ref for cl->leaf.q, the other for dev_queue->qdisc. */
 		qdisc_refcount_inc(new);
 		old_q = htb_graft_helper(dev_queue, new);
···
 		new_q = qdisc_create_dflt(dev_queue, &pfifo_qdisc_ops,
 					  cl->parent->common.classid,
 					  NULL);
-		if (q->offload) {
-			if (new_q)
-				htb_set_lockdep_class_child(new_q);
+		if (q->offload)
 			htb_parent_to_leaf_offload(sch, dev_queue, new_q);
-		}
 	}
 
 	sch_tree_lock(sch);
···
 		new_q = qdisc_create_dflt(dev_queue, &pfifo_qdisc_ops,
 					  classid, NULL);
 		if (q->offload) {
-			if (new_q) {
-				htb_set_lockdep_class_child(new_q);
-				/* One ref for cl->leaf.q, the other for
-				 * dev_queue->qdisc.
-				 */
+			/* One ref for cl->leaf.q, the other for dev_queue->qdisc. */
+			if (new_q)
 				qdisc_refcount_inc(new_q);
-			}
 			old_q = htb_graft_helper(dev_queue, new_q);
 			/* No qdisc_put needed. */
 			WARN_ON(!(old_q->flags & TCQ_F_BUILTIN));