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Date:   Wed, 17 May 2023 14:18:28 -0700
From:   Peilin Ye <yepeilin.cs@gmail.com>
To:     Jakub Kicinski <kuba@kernel.org>
Cc:     "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Paolo Abeni <pabeni@redhat.com>,
        Jamal Hadi Salim <jhs@mojatatu.com>,
        Cong Wang <xiyou.wangcong@gmail.com>,
        Jiri Pirko <jiri@resnulli.us>,
        Peilin Ye <peilin.ye@bytedance.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        John Fastabend <john.fastabend@gmail.com>,
        Vlad Buslov <vladbu@mellanox.com>,
        Pedro Tammela <pctammela@mojatatu.com>,
        Hillf Danton <hdanton@sina.com>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, Cong Wang <cong.wang@bytedance.com>
Subject: Re: [PATCH net 6/6] net/sched: qdisc_destroy() old ingress and
 clsact Qdiscs before grafting
Message-ID: <ZGVEpHMkerjA2+0V@C02FL77VMD6R>
References: <cover.1683326865.git.peilin.ye@bytedance.com>
 <e6c4681dd9205d702ae2e6124e20c6210520e76e.1683326865.git.peilin.ye@bytedance.com>
 <20230517114825.5d7c85a4@kernel.org>
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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