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

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Date: Wed, 17 May 2023 11:48:25 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Peilin Ye <yepeilin.cs@...il.com>
Cc: "David S. Miller" <davem@...emloft.net>, Eric Dumazet
 <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, Jamal Hadi Salim
 <jhs@...atatu.com>, Cong Wang <xiyou.wangcong@...il.com>, Jiri Pirko
 <jiri@...nulli.us>, Peilin Ye <peilin.ye@...edance.com>, Daniel Borkmann
 <daniel@...earbox.net>, John Fastabend <john.r.fastabend@...el.com>, Vlad
 Buslov <vladbu@...lanox.com>, Pedro Tammela <pctammela@...atatu.com>, Hillf
 Danton <hdanton@...a.com>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, Cong Wang <cong.wang@...edance.com>
Subject: Re: [PATCH net 6/6] net/sched: qdisc_destroy() old ingress and
 clsact Qdiscs before grafting

On Fri,  5 May 2023 17:16:10 -0700 Peilin Ye 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?