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

After commit 95dc19299f74 ("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>

2 files changed, 5 insertions, 2 deletions

diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c
index 547b4a88ae2a..c31190e29b90 100644
--- a/net/sched/sch_api.c
+++ b/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);
 
diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 6a91d7d48ade..32bb942d2faa 100644
--- a/net/sched/sch_generic.c
+++ b/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);
                 }
         }
 }