Re: [PATCH] init: Add lockdep annotation to kthreadd_done completer

[Helge Deller]

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@xxxxxx> 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