From: Petr Machata <petrm@mellanox.com>
Date: Mon, 6 Jan 2020 18:01:56 +0000
Subject: net: sch_prio: When ungrafting, replace with FIFO
Git-commit: 240ce7f6428ff5188b9eedc066e1e4d645b8635f
Patch-mainline: 5.5-rc6
References: networking-stable-20_01_11

When a child Qdisc is removed from one of the PRIO Qdisc's bands, it is
replaced unconditionally by a NOOP qdisc. As a result, any traffic hitting
that band gets dropped. That is incorrect--no Qdisc was explicitly added
when PRIO was created, and after removal, none should have to be added
either.

Fix PRIO by first attempting to create a default Qdisc and only falling
back to noop when that fails. This pattern of attempting to create an
invisible FIFO, using NOOP only as a fallback, is also seen in other
Qdiscs.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Petr Machata <petrm@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
 net/sched/sch_prio.c |   10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

--- a/net/sched/sch_prio.c
+++ b/net/sched/sch_prio.c
@@ -244,8 +244,14 @@ static int prio_graft(struct Qdisc *sch,
 	bool any_qdisc_is_offloaded;
 	int err;
 
-	if (new == NULL)
-		new = &noop_qdisc;
+	if (!new) {
+		new = qdisc_create_dflt(sch->dev_queue, &pfifo_qdisc_ops,
+					TC_H_MAKE(sch->handle, arg), extack);
+		if (!new)
+			new = &noop_qdisc;
+		else
+			qdisc_hash_add(new, true);
+	}
 
 	*old = qdisc_replace(sch, new, &q->queues[band]);