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

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Date: Mon, 15 May 2023 15:45:15 -0700
From: Peilin Ye <yepeilin.cs@...il.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Jiri Pirko <jiri@...nulli.us>, Daniel Borkmann <daniel@...earbox.net>,
	"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>,
	Peilin Ye <peilin.ye@...edance.com>,
	John Fastabend <john.fastabend@...il.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 Thu, May 11, 2023 at 05:11:23PM -0700, Peilin Ye wrote:
> > > ->init() may be too early, aren't there any error points which could
> > > prevent the Qdisc from binding after ->init() was called?
> >
> > You're right, it's in qdisc_create(), argh...
>
> ->destroy() is called for all error points between ->init() and
> dev_graft_qdisc().  I'll try handling it in ->destroy().

Sorry for any confusion: there is no point at all undoing "setting dev
pointer to b1" in ->destroy() because datapath has already been affected.

To summarize, grafting B mustn't fail after setting dev pointer to b1, so
->init() is too early, because e.g. if user requested [1] to create a rate
estimator, gen_new_estimator() could fail after ->init() in
qdisc_create().

On the other hand, ->attach() is too late because it's later than
dev_graft_qdisc(), so concurrent filter requests might see uninitialized
dev pointer in theory.

Please suggest; is adding another callback (or calling ->attach()) right
before dev_graft_qdisc() for ingress (clsact) Qdiscs too much for this
fix?

[1] e.g. $ tc qdisc add dev eth0 estimator 1s 8s clsact

Thanks,
Peilin Ye