[PATCH v3 net-next 03/10] net/sched: taprio: try again to report q->qdiscs[] to qdisc_leaf()

From: Vladimir Oltean
Date: Tue Aug 01 2023 - 14:25:02 EST

This is another stab at commit 1461d212ab27 ("net/sched: taprio: make
qdisc_leaf() see the per-netdev-queue pfifo child qdiscs"), later
reverted in commit af7b29b1deaa ("Revert "net/sched: taprio: make
qdisc_leaf() see the per-netdev-queue pfifo child qdiscs"").

I believe that the problems that caused the revert were fixed, and thus,
this change is identical to the original patch.

Its purpose is to properly reject attaching a software taprio child
qdisc to a software taprio parent. Because unoffloaded taprio currently
reports itself (the root Qdisc) as the return value from qdisc_leaf(),
then the process of attaching another taprio as child to a Qdisc class
of the root will just result in a Qdisc_ops :: change() call for the
root. Whereas that's not we want. We want Qdisc_ops :: init() to be
called for the taprio child, in order to give the taprio child a chance
to check whether its sch->parent is TC_H_ROOT or not (and reject this

Signed-off-by: Vladimir Oltean <vladimir.oltean@xxxxxxx>
v2->v3: none
v1->v2: rebase on top of rtnl_dereference() change for txq->qdisc_sleeping

net/sched/sch_taprio.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
index 41944197876a..c9ad585b5dc7 100644
--- a/net/sched/sch_taprio.c
+++ b/net/sched/sch_taprio.c
@@ -2442,12 +2442,14 @@ static int taprio_dump(struct Qdisc *sch, struct sk_buff *skb)

static struct Qdisc *taprio_leaf(struct Qdisc *sch, unsigned long cl)
- struct netdev_queue *dev_queue = taprio_queue_get(sch, cl);
+ struct taprio_sched *q = qdisc_priv(sch);
+ struct net_device *dev = qdisc_dev(sch);
+ unsigned int ntx = cl - 1;

- if (!dev_queue)
+ if (ntx >= dev->num_tx_queues)
return NULL;

- return rtnl_dereference(dev_queue->qdisc_sleeping);
+ return q->qdiscs[ntx];

static unsigned long taprio_find(struct Qdisc *sch, u32 classid)