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

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Date: Thu, 11 May 2023 16:46:33 -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 04:20:23PM -0700, Jakub Kicinski wrote:
> > But I see your point, thanks for the suggestion!  I'll try ->init() and
> > create v2.
>
> ->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...

On Thu, May 11, 2023 at 04:20:23PM -0700, Jakub Kicinski wrote:
> > > > Looking at the code, I think there is no guarantee that (1st) cannot
> > > > happen after (2nd), although unlikely?  Can RTNL-lockless RTM_NEWTFILTER
> > > > handlers get preempted?
> > >
> > > Right, we need qdisc_graft(B) to update the appropriate dev pointer
> > > to point to b1. With that the ordering should not matter. Probably
> > > using the ->attach() callback?
> >
> > ->attach() is later than dev_graft_qdisc().  We should get ready for
> > concurrent filter requests (i.e. have dev pointer pointing to b1) before
> > grafting (publishing) B.
>
> I thought even for "unlocked" filter operations the start of it is
> under the lock, but the lock gets dropped after qdisc/block are found.
> I could be misremembering, I haven't looked at the code.

No, f.e. RTM_NEWTFILTER is registered as RTNL_FLAG_DOIT_UNLOCKED, so
tc_new_tfilter() starts and calls __tcf_qdisc_find() without RTNL mutex,
at least in latest code.

Thinking,
Peilin Ye