netdev - Re: [PATCH net 1/2] net/sched: ets: Remove drr class from the active list if it changes to strict

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Message-ID: <87ikefwybc.fsf@nvidia.com>
Date: Tue, 9 Dec 2025 14:42:10 +0100
From: Petr Machata <petrm@...dia.com>
To: Victor Nogueira <victor@...atatu.com>
CC: <davem@...emloft.net>, <kuba@...nel.org>, <pabeni@...hat.com>,
	<edumazet@...gle.com>, <jhs@...atatu.com>, <jiri@...nulli.us>,
	<xiyou.wangcong@...il.com>, <horms@...nel.org>, <dcaratti@...hat.com>,
	<petrm@...dia.com>, <netdev@...r.kernel.org>
Subject: Re: [PATCH net 1/2] net/sched: ets: Remove drr class from the
 active list if it changes to strict


Victor Nogueira <victor@...atatu.com> writes:

> Whenever a user issues an ets qdisc change command, transforming a
> drr class into a strict one, the ets code isn't checking whether that
> class was in the active list and removing it. This means that, if a
> user changes a strict class (which was in the active list) back to a drr
> one, that class will be added twice to the active list [1].
>
> Doing so with the following commands:
>
> tc qdisc add dev lo root handle 1: ets bands 2 strict 1
> tc qdisc add dev lo parent 1:2 handle 20: \
>     tbf rate 8bit burst 100b latency 1s
> tc filter add dev lo parent 1: basic classid 1:2
> ping -c1 -W0.01 -s 56 127.0.0.1
> tc qdisc change dev lo root handle 1: ets bands 2 strict 2
> tc qdisc change dev lo root handle 1: ets bands 2 strict 1
> ping -c1 -W0.01 -s 56 127.0.0.1
>
> Will trigger the following splat with list debug turned on:
>
> [   59.279014][  T365] ------------[ cut here ]------------
> [   59.279452][  T365] list_add double add: new=ffff88801d60e350, prev=ffff88801d60e350, next=ffff88801d60e2c0.
> [   59.280153][  T365] WARNING: CPU: 3 PID: 365 at lib/list_debug.c:35 __list_add_valid_or_report+0x17f/0x220
> [   59.280860][  T365] Modules linked in:
> [   59.281165][  T365] CPU: 3 UID: 0 PID: 365 Comm: tc Not tainted 6.18.0-rc7-00105-g7e9f13163c13-dirty #239 PREEMPT(voluntary)
> [   59.281977][  T365] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
> [   59.282391][  T365] RIP: 0010:__list_add_valid_or_report+0x17f/0x220
> [   59.282842][  T365] Code: 89 c6 e8 d4 b7 0d ff 90 0f 0b 90 90 31 c0 e9 31 ff ff ff 90 48 c7 c7 e0 a0 22 9f 48 89 f2 48 89 c1 4c 89 c6 e8 b2 b7 0d ff 90 <0f> 0b 90 90 31 c0 e9 0f ff ff ff 48 89 f7 48 89 44 24 10 4c 89 44
> ...
> [   59.288812][  T365] Call Trace:
> [   59.289056][  T365]  <TASK>
> [   59.289224][  T365]  ? srso_alias_return_thunk+0x5/0xfbef5
> [   59.289546][  T365]  ets_qdisc_change+0xd2b/0x1e80
> [   59.289891][  T365]  ? __lock_acquire+0x7e7/0x1be0
> [   59.290223][  T365]  ? __pfx_ets_qdisc_change+0x10/0x10
> [   59.290546][  T365]  ? srso_alias_return_thunk+0x5/0xfbef5
> [   59.290898][  T365]  ? __mutex_trylock_common+0xda/0x240
> [   59.291228][  T365]  ? __pfx___mutex_trylock_common+0x10/0x10
> [   59.291655][  T365]  ? srso_alias_return_thunk+0x5/0xfbef5
> [   59.291993][  T365]  ? srso_alias_return_thunk+0x5/0xfbef5
> [   59.292313][  T365]  ? trace_contention_end+0xc8/0x110
> [   59.292656][  T365]  ? srso_alias_return_thunk+0x5/0xfbef5
> [   59.293022][  T365]  ? srso_alias_return_thunk+0x5/0xfbef5
> [   59.293351][  T365]  tc_modify_qdisc+0x63a/0x1cf0
>
> Fix this by always checking and removing an ets class from the active list
> when changing it to strict.
>
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git/tree/net/sched/sch_ets.c?id=ce052b9402e461a9aded599f5b47e76bc727f7de#n663
>
> Fixes: cd9b50adc6bb9 ("net/sched: ets: fix crash when flipping from 'strict' to 'quantum'")
> Acked-by: Jamal Hadi Salim <jhs@...atatu.com>
> Signed-off-by: Victor Nogueira <victor@...atatu.com>

Reviewed-by: Petr Machata <petrm@...dia.com>