netdev - Re: [PATCH net 5/6] net/sched: fix packet loop on netem when duplicate is on

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Message-ID: <CAM0EoMn7Mza5LqV5f6MMgacuELncbr1Ka6BOi7SA_2Fe3a7LCA@mail.gmail.com>
Date: Sun, 11 Jan 2026 16:56:49 -0500
From: Jamal Hadi Salim <jhs@...atatu.com>
To: Cong Wang <xiyou.wangcong@...il.com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, 
	pabeni@...hat.com, horms@...nel.org, andrew+netdev@...n.ch, 
	netdev@...r.kernel.org, jiri@...nulli.us, victor@...atatu.com, 
	dcaratti@...hat.com, lariel@...dia.com, daniel@...earbox.net, 
	pablo@...filter.org, kadlec@...filter.org, fw@...len.de, phil@....cc, 
	netfilter-devel@...r.kernel.org, coreteam@...filter.org, 
	zyc199902@...omail.cn, lrGerlinde@...lfence.com, jschung2@...ton.me, 
	William Liu <will@...lsroot.io>, Savino Dicanosa <savy@...t3mfailure.io>
Subject: Re: [PATCH net 5/6] net/sched: fix packet loop on netem when
 duplicate is on

On Sun, Jan 11, 2026 at 3:39 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
>
> On Sun, Jan 11, 2026 at 8:40 AM Jamal Hadi Salim <jhs@...atatu.com> wrote:
> > -               q->duplicate = 0;
> > +               skb2->ttl++; /* prevent duplicating a dup... */
> >                 rootq->enqueue(skb2, rootq, to_free);
> > -               q->duplicate = dupsave;
>
> As I already explained many times, the ROOT cause is enqueuing
> to the root qdisc, not anything else.
>
> We need to completely forget all the kernel knowledge and ask
> a very simple question here: is enqueuing to root qdisc a reasonable
> use? More importantly, could we really define it?
>
> I already provided my answer in my patch description, sorry for not
> keeping repeating it for at least the 3rd time.
>
> Therefore, I still don't think you fix the root cause here. The
> problematic behavior of enqueuing to root qdisc should be corrected,
> regardless of any kernel detail.
>

The root cause is a loop in the existing code, present since the
duplication feature was introduced into netem about 20 years ago. That
code enqueues to the root qdisc.

cheers,
jamal