linux-kernel - Re: [REGRESSION] 6.14.3 panic - kernel NULL pointer dereference in htb

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Message-ID: <20250421131000.6299a8e0@frodo.int.wylie.me.uk>
Date: Mon, 21 Apr 2025 13:10:00 +0100
From: "Alan J. Wylie" <alan@...ie.me.uk>
To: Holger Hoffstätte <holger@...lied-asynchrony.com>
Cc: Jamal Hadi Salim <jhs@...atatu.com>, regressions@...ts.linux.dev, Cong
 Wang <xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org, Octavian Purdila
 <tavip@...gle.com>, Toke Høiland-Jørgensen
 <toke@...hat.com>, stable@...r.kernel.org
Subject: Re: [REGRESSION] 6.14.3 panic - kernel NULL pointer dereference in
 htb_dequeue

On Mon, 21 Apr 2025 13:50:52 +0200
Holger Hoffstätte <holger@...lied-asynchrony.com> wrote:

> On 2025-04-21 11:40, Alan J. Wylie wrote:
> > #regzbot introduced: 6.14.2..6.14.3
> > 
> > Since 6.14.3 I have been seeing random panics, all in htb_dequeue.
> > 6.14.2 was fine.  
> 
> 6.14.3 contains:
> "codel: remove sch->q.qlen check before qdisc_tree_reduce_backlog()"
> aka https://web.git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/net/sched?h=linux-6.14.y&id=a57fe60ef4cf96bfbb6b58397ec28bdb5a5c6b31
> 
> Is your HTB backed by fq_codel by any chance?

Yes

# grep fq 91-tc.sh 
quantum=300		# fq_codel quantum 300 gives a boost to interactive flows
modprobe sch_fq_codel
tc qdisc add dev "$ext_ingress" parent 1:11 fq_codel quantum $quantum ecn
tc qdisc add dev "$ext" parent 1:11 fq_codel quantum $quantum noecn

> If so, try either reverting the above or adding:
> "sch_htb: make htb_qlen_notify() idempotent" aka
> https://web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=5ba8b837b522d7051ef81bacf3d95383ff8edce5
> 
> which was successfully not added to 6.14.3, along with the rest of the
> series:
> https://lore.kernel.org/all/20250403211033.166059-2-xiyou.wangcong@gmail.com/

"successfully not added"?

$ git cherry-pick  5ba8b837b522d7051ef81bacf3d95383ff8edce5
[linux-6.14.y 2285c724bf7d] sch_htb: make htb_qlen_notify() idempotent
 Author: Cong Wang <xiyou.wangcong@...il.com>
 Date: Thu Apr 3 14:10:23 2025 -0700
 1 file changed, 2 insertions(+)

It will take a while (perhaps days?) before I can confirm success.

Thanks
Alan

-- 
Alan J. Wylie     https://www.wylie.me.uk/     mailto:<alan@...ie.me.uk>

Dance like no-one's watching. / Encrypt like everyone is.
Security is inversely proportional to convenience