net: sch_prio: When ungrafting, replace with FIFO

[ Upstream commit 240ce7f6428ff5188b9eedc066e1e4d645b8635f ]

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: 1da177e ("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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

net/sched/sch_prio.c

@@ -232,8 +232,14 @@ static int prio_graft(struct Qdisc *sch, unsigned long arg, struct Qdisc *new,
 	struct prio_sched_data *q = qdisc_priv(sch);
 	unsigned long band = arg - 1;
 
-	if (new == NULL)
-		new = &noop_qdisc;
+	if (!new) {
+		new = qdisc_create_dflt(sch->dev_queue, &pfifo_qdisc_ops,
+					TC_H_MAKE(sch->handle, arg));
+		if (!new)
+			new = &noop_qdisc;
+		else
+			qdisc_hash_add(new);
+	}
 
 	*old = qdisc_replace(sch, new, &q->queues[band]);
 	return 0;