Merge branch 'net-ungraft-prio'

Petr Machata says:

====================
When ungrafting from PRIO, replace child with FIFO

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.

In patch #2, this problem is fixed for 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 some other Qdiscs.

The only driver currently offloading PRIO (and thus presumably the only one
impacted by this) is mlxsw. Therefore patch #1 extends mlxsw to handle the
replacement by an invisible FIFO gracefully.
====================
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlxsw/spectrum_qdisc.c

@@ -651,6 +651,13 @@ mlxsw_sp_qdisc_prio_graft(struct mlxsw_sp_port *mlxsw_sp_port,
 	    mlxsw_sp_port->tclass_qdiscs[tclass_num].handle == p->child_handle)
 		return 0;
 
+	if (!p->child_handle) {
+		/* This is an invisible FIFO replacing the original Qdisc.
+		 * Ignore it--the original Qdisc's destroy will follow.
+		 */
+		return 0;
+	}
+
 	/* See if the grafted qdisc is already offloaded on any tclass. If so,
 	 * unoffload it.
 	 */

net/sched/sch_prio.c

@@ -292,8 +292,14 @@ static int prio_graft(struct Qdisc *sch, unsigned long arg, struct Qdisc *new,
 	struct tc_prio_qopt_offload graft_offload;
 	unsigned long band = arg - 1;
 
-	if (new == NULL)
+	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]);