Re: [syzbot] [kernel?] WARNING in _cpu_down

[Thomas Gleixner]

On Mon, Mar 09 2026 at 10:57, syzbot wrote:
> syzbot found the following issue on:
>
> HEAD commit:    4ae12d8bd9a8 Merge tag 'kbuild-fixes-7.0-2' of git://git.k..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=177cca02580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=163cf0fb07ea84d3
> dashboard link: https://syzkaller.appspot.com/bug?extid=123e1b70473ce213f3af
> compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
> smpboot: CPU 1 is now offline
> smpboot: Booting Node 0 Processor 1 APIC 0x1
> ------------[ cut here ]------------
> DEAD callback error for CPU1
> WARNING: kernel/cpu.c:1463 at _cpu_down+0x759/0x1020 kernel/cpu.c:1463, CPU#0: syz.0.1960/14614

That means one of the hotplug state callbacks which are issued after a
CPU is dead returned an error. That's wrong. Those callbacks are not
supposed to fail as there is no way to rollback at that point.

A quick coccinelle search finds exactly one callback in that stage which
might return an error code:

      padata_cpu_dead()

Not sure how to fix that. I leave it to the padata folks ...

Thanks

        tglx