Subject: Re: [syzbot] [kernel?] WARNING: ODEBUG bug in smpboot_thread_fn

From: Thomas Gleixner <[email-masked]>
Date: Wed, 06 May 2026 18:29:43 +0200

On Mon, May 04 2026 at 05:23, syzbot wrote:

Cc'ed network/bridge people as that's clearly in their realm.

> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    6d35786de281 Merge tag 'for-linus' of git://git.kernel.org..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=126bf21f980000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=f2e8ebfec4636d32
> dashboard link: https://syzkaller.appspot.com/bug?extid=ae231e0552fa77b26ea1
> compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/b3ab34c4c807/disk-6d35786d.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/03f465478178/vmlinux-6d35786d.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/29d261604425/bzImage-6d35786d.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: [email-masked]
>
> ------------[ cut here ]------------
> ODEBUG: free active (active state 0) object: ffff888033a47278 object type: timer_list hint: br_ip6_multicast_port_query_expired+0x0/0x380 net/bridge/br_multicast.c:-1

                                                                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
An object which contains an active timer is RCU freed....

> WARNING: lib/debugobjects.c:632 at debug_print_object lib/debugobjects.c:629 [inline], CPU#1: rcuc/1/28
> WARNING: lib/debugobjects.c:632 at __debug_check_no_obj_freed lib/debugobjects.c:1116 [inline], CPU#1: rcuc/1/28
> WARNING: lib/debugobjects.c:632 at debug_check_no_obj_freed+0x405/0x550 lib/debugobjects.c:1146, CPU#1: rcuc/1/28
> Modules linked in:
> CPU: 1 UID: 0 PID: 28 Comm: rcuc/1 Tainted: G             L      syzkaller #0 PREEMPT_{RT,(full)} 
> Tainted: [L]=SOFTLOCKUP
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
> RIP: 0010:debug_print_object lib/debugobjects.c:629 [inline]
> RIP: 0010:__debug_check_no_obj_freed lib/debugobjects.c:1116 [inline]
> RIP: 0010:debug_check_no_obj_freed+0x44a/0x550 lib/debugobjects.c:1146
> Code: 89 44 24 20 e8 27 81 7f fd 48 8b 44 24 20 4c 8b 4d 00 4c 89 ef 48 c7 c6 a0 53 a7 8b 48 c7 c2 20 59 a7 8b 8b 0c 24 4d 89 f8 50 <67> 48 0f b9 3a 48 83 c4 08 4c 8b 6c 24 18 48 b9 00 00 00 00 00 fc
> RSP: 0018:ffffc90000a2fa90 EFLAGS: 00010246
> RAX: ffffffff89f32240 RBX: ffffffff99a8edc0 RCX: 0000000000000000
> RDX: ffffffff8ba75920 RSI: ffffffff8ba753a0 RDI: ffffffff8f933740
> RBP: ffffffff8b4f5380 R08: ffff888033a47278 R09: ffffffff8b4f6700
> R10: dffffc0000000000 R11: ffffffff81b0c180 R12: ffff888033a47400
> R13: ffffffff8f933740 R14: ffff888033a47000 R15: ffff888033a47278
> FS:  0000000000000000(0000) GS:ffff888126279000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007fb873a4da08 CR3: 000000004cc78000 CR4: 00000000003526f0
> Call Trace:
>  <TASK>
>  slab_free_hook mm/slub.c:2620 [inline]
>  slab_free mm/slub.c:6250 [inline]
>  kfree+0x13e/0x6c0 mm/slub.c:6565
>  kobject_cleanup lib/kobject.c:689 [inline]
>  kobject_release lib/kobject.c:720 [inline]
>  kref_put include/linux/kref.h:65 [inline]
>  kobject_put+0x228/0x560 lib/kobject.c:737
>  rcu_do_batch kernel/rcu/tree.c:2617 [inline]
>  rcu_core kernel/rcu/tree.c:2869 [inline]
>  rcu_cpu_kthread+0x99e/0x1470 kernel/rcu/tree.c:2957
>  smpboot_thread_fn+0x541/0xa50 kernel/smpboot.c:160
>  kthread+0x388/0x470 kernel/kthread.c:436
>  ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>  </TASK>