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Date:   Thu, 4 May 2023 21:22:46 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Wander Lairson Costa <wander@redhat.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
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Subject: Re: [PATCH v7 2/3] sched/task: Add the put_task_struct_atomic_safe()
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On 05/04, Wander Lairson Costa wrote:
>
> On Thu, May 4, 2023 at 12:23 PM Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > Yes, but as Sebastian explained CONFIG_PROVE_RAW_LOCK_NESTING won't like it.
> >
> >         https://lore.kernel.org/all/Y+zFNrCjBn53%2F+Q2@linutronix.de/
> >
> 
> I think that was my confusion in that thread. My understanding is that
> CONFIG_PROVE_RAW_LOCK_NESTING will check lock ordering but not
> context.

Sorry, I don't understand... perhaps I missed something. But iiuc
the problem is simple.

So, this code

	raw_spin_lock(one);
	spin_lock(two);

is obviously wrong if CONFIG_PREEMPT_RT.

Without PREEMPT_RT this code is fine because raw_spinlock_t and spinlock_t
are the same thing. Except they have different lockdep annotations if
CONFIG_PROVE_RAW_LOCK_NESTING is true, LD_WAIT_SPIN and LD_WAIT_CONFIG.

So if CONFIG_PROVE_RAW_LOCK_NESTING is set, lockdep will complain even
on the !PREEMPT_RT kernel, iow it checks the nesting as if the code runs
on with PREEMPT_RT.

Cough... not sure my explanation can help ;) It looks very confusing when
I read it.

Oleg.