Re: [PATCH net 6/6] net/sched: qdisc_destroy() old ingress and clsact Qdiscs before grafting

[Peilin Ye <yepeilin.cs@gmail.com>]

On Wed, May 17, 2023 at 11:48:25AM -0700, Jakub Kicinski wrote:
> >             } else {
> > -                   dev_queue = dev_ingress_queue(dev);
> > -                   old = dev_graft_qdisc(dev_queue, new);
> > +                   old = dev_graft_qdisc(dev_queue, NULL);
> > +
> > +                   /* {ingress,clsact}_destroy() "old" before grafting "new" to avoid
> > +                    * unprotected concurrent accesses to net_device::miniq_{in,e}gress
> > +                    * pointer(s) in mini_qdisc_pair_swap().
> > +                    */
> > +                   qdisc_notify(net, skb, n, classid, old, new, extack);
> > +                   qdisc_destroy(old);
> > +
> > +                   dev_graft_qdisc(dev_queue, new);
>
> BTW can't @old be NULL here?

ingress_queue->qdisc_sleeping is initialized to &noop_qdisc (placeholder)
in dev_ingress_queue_create(), and dev_graft_qdisc() also grafts
&noop_qdisc to represent "there's no Qdisc":

	/* ... and graft new one */
	if (qdisc == NULL)
		qdisc = &noop_qdisc;
	dev_queue->qdisc_sleeping = qdisc;

So @old can't be NULL here.

Thanks,
Peilin Ye