pkt_sched: set root qdisc before change() in attach_default_qdiscs()

After commit 95dc1929 ("pkt_sched: give visibility to mq slave
qdiscs") we call disc_list_add() while the device qdisc might be
the noop_qdisc one.

This shows up as duplicates in "tc qdisc show", as all inactive devices
point to noop_qdisc.

Fix this by setting dev->qdisc to the new qdisc before calling
ops->change() in attach_default_qdiscs()

Add a WARN_ON_ONCE() to catch any future similar problem.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_api.c

@@ -273,8 +273,11 @@ static struct Qdisc *qdisc_match_from_root(struct Qdisc *root, u32 handle)
 
 void qdisc_list_add(struct Qdisc *q)
 {
+	struct Qdisc *root = qdisc_dev(q)->qdisc;
+
+	WARN_ON_ONCE(root == &noop_qdisc);
 	if ((q->parent != TC_H_ROOT) && !(q->flags & TCQ_F_INGRESS))
-		list_add_tail(&q->list, &qdisc_dev(q)->qdisc->list);
+		list_add_tail(&q->list, &root->list);
 }
 EXPORT_SYMBOL(qdisc_list_add);
 

net/sched/sch_generic.c

@@ -718,8 +718,8 @@ static void attach_default_qdiscs(struct net_device *dev)
 	} else {
 		qdisc = qdisc_create_dflt(txq, &mq_qdisc_ops, TC_H_ROOT);
 		if (qdisc) {
-			qdisc->ops->attach(qdisc);
 			dev->qdisc = qdisc;
+			qdisc->ops->attach(qdisc);
 		}
 	}
 }