From mboxrd@z Thu Jan  1 00:00:00 1970
From: peterz@infradead.org
Subject: Re: [PATCH 1/2] lockdep: improve current->(hard|soft)irqs_enabled
 synchronisation with actual irq state
Date: Sun, 26 Jul 2020 13:59:44 +0200
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To: Nicholas Piggin <npiggin@gmail.com>
Cc: Alexey Kardashevskiy <aik@ozlabs.ru>, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>

On Sun, Jul 26, 2020 at 02:14:34PM +1000, Nicholas Piggin wrote:
> > Now, x86, and at least arm64 call nmi_enter() before
> > trace_hardirqs_off(), but AFAICT Power never did that, and that's part
> > of the problem. nmi_enter() does lockdep_off() and that _used_ to also
> > kill IRQ tracking.
> 
> Power does do nmi_enter -- __perf_event_interrupt()
> 
>         nmi = perf_intr_is_nmi(regs);
>         if (nmi)
>                 nmi_enter();
>         else
>                 irq_enter();

That's _waaaay_ too late, you've done the trace_hardirqs_{off,on} in
arch/powerpc/kernel/exceptions-64e.S, you need to _first_ do nmi_enter()
and _then_ trace_hardirqs_off(), or rather, that's still broken, but
then we get into the details of entry ordering.