linux-kernel - Re: [PATCH] init: Add lockdep annotation to kthreadd

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Date:   Sat, 12 Aug 2023 16:02:10 +0200
From:   Helge Deller <deller@....de>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-parisc@...r.kernel.org,
        Mike Rapoport <rppt@...nel.org>
Subject: Re: [PATCH] init: Add lockdep annotation to kthreadd_done completer

Hi Andrew,

On 8/11/23 19:44, Helge Deller wrote:
> On 8/11/23 18:43, Andrew Morton wrote:
>> On Fri, 11 Aug 2023 18:04:22 +0200 Helge Deller <deller@....de> wrote:
>>
>>> Add the missing lockdep annotation to avoid this warning:
>>>
>>>   INFO: trying to register non-static key.
>>>   The code is fine but needs lockdep annotation, or maybe
>>>   you didn't initialize this object before use?
>>>   turning off the locking correctness validator.
>>>   CPU: 0 PID: 1 Comm: swapper/0 Not tainted 6.5.0-rc5+ #681
>>>   Hardware name: 9000/785/C3700
>>>   Backtrace:
>>>    [<000000004030bcd0>] show_stack+0x74/0xb0
>>>    [<0000000041469c7c>] dump_stack_lvl+0x104/0x180
>>>    [<0000000041469d2c>] dump_stack+0x34/0x48
>>>    [<000000004040e5b4>] register_lock_class+0xd24/0xd30
>>>    [<000000004040c21c>] __lock_acquire.isra.0+0xb4/0xac8
>>>    [<000000004040cd60>] lock_acquire+0x130/0x298
>>>    [<000000004146df54>] _raw_spin_lock_irq+0x60/0xb8
>>>    [<0000000041472044>] wait_for_completion+0xa0/0x2d0
>>>    [<000000004146b544>] kernel_init+0x48/0x3a8
>>>    [<0000000040302020>] ret_from_kernel_thread+0x20/0x28
>>>
>>> ...
>>>
>>> --- a/init/main.c
>>> +++ b/init/main.c
>>> @@ -682,6 +682,8 @@ noinline void __ref __noreturn rest_init(void)
>>>       struct task_struct *tsk;
>>>       int pid;
>>>
>>> +    init_completion(&kthreadd_done);
>>> +
>>>       rcu_scheduler_starting();
>>>       /*
>>>        * We need to spawn init first so that it obtains pid 1, however
>>
>> This is pretty old code, as is the page_alloc_init_late() change.  Do
>> we know why this warning has just turned up lately?

[dropped Helge's assumptions that it's related to parisc's unusual
__ARCH_SPIN_LOCK_UNLOCKED_VAL value. It turned out to be wrong.

Now I was able to trace down why I see those lockdep warnings on parisc.
The short answer is:
On parisc the _initdata section lies outside of the usual kernel
_stext ... _end range. Lockdep calls static_obj() which currently assumes
that __initdata is inside that range and thus returns "false".
That's why lockdep then reports
	INFO: trying to register non-static key.
which is wrong.

Please drop those 3 lockdep patches from your mm-queue:

mm-add-lockdep-annotation-to-pgdat_init_all_done_comp-completer.patch
init-add-lockdep-annotation-to-kthreadd_done-completer.patch
watchdog-fix-lockdep-warning.patch

I'll send one single patch which fixes static_obj() instead.

Thanks,
Helge