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

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Message-ID: <20230516122205.6f198c3e@kernel.org>
Date: Tue, 16 May 2023 12:22:05 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Vlad Buslov <vladbu@...dia.com>
Cc: Peilin Ye <yepeilin.cs@...il.com>, 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>,
 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 Mon, 15 May 2023 15:45:15 -0700 Peilin Ye wrote:
> On Thu, May 11, 2023 at 05:11:23PM -0700, Peilin Ye wrote:
> > > 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

Vlad, could you please clarify how you expect the unlocked filter
operations to work when the qdisc has global state?