net: sched: Add multi-queue support to sch_tree_lock

The existing qdiscs that set TCQ_F_MQROOT don't use sch_tree_lock.
However, hardware-offloaded HTB will start setting this flag while also
using sch_tree_lock.

The current implementation of sch_tree_lock basically locks on
qdisc->dev_queue->qdisc, and it works fine when the tree is attached to
some queue. However, it's not the case for MQROOT qdiscs: such a qdisc
is the root itself, and its dev_queue just points to queue 0, while not
actually being used, because there are real per-queue qdiscs.

This patch changes the logic of sch_tree_lock and sch_tree_unlock to
lock the qdisc itself if it's the MQROOT.

Signed-off-by: Maxim Mikityanskiy <maximmi@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/sch_generic.h

@@ -552,14 +552,20 @@ static inline struct net_device *qdisc_dev(const struct Qdisc *qdisc)
 	return qdisc->dev_queue->dev;
 }
 
-static inline void sch_tree_lock(const struct Qdisc *q)
+static inline void sch_tree_lock(struct Qdisc *q)
 {
-	spin_lock_bh(qdisc_root_sleeping_lock(q));
+	if (q->flags & TCQ_F_MQROOT)
+		spin_lock_bh(qdisc_lock(q));
+	else
+		spin_lock_bh(qdisc_root_sleeping_lock(q));
 }
 
-static inline void sch_tree_unlock(const struct Qdisc *q)
+static inline void sch_tree_unlock(struct Qdisc *q)
 {
-	spin_unlock_bh(qdisc_root_sleeping_lock(q));
+	if (q->flags & TCQ_F_MQROOT)
+		spin_unlock_bh(qdisc_lock(q));
+	else
+		spin_unlock_bh(qdisc_root_sleeping_lock(q));
 }
 
 extern struct Qdisc noop_qdisc;