linux-kernel - Re: [PATCH 1/2] lockdep: improve current->(hard|soft)irqs_enabled synchronisation with actual irq state

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Message-Id: <1597793862.l8c4pmmzpq.astroid@bobo.none>
Date:   Wed, 19 Aug 2020 09:54:57 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     peterz@...radead.org
Cc:     Alexey Kardashevskiy <aik@...abs.ru>, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>
Subject: Re: [PATCH 1/2] lockdep: improve current->(hard|soft)irqs_enabled
 synchronisation with actual irq state

Excerpts from peterz@...radead.org's message of August 19, 2020 1:41 am:
> On Tue, Aug 18, 2020 at 05:22:33PM +1000, Nicholas Piggin wrote:
>> Excerpts from peterz@...radead.org's message of August 12, 2020 8:35 pm:
>> > On Wed, Aug 12, 2020 at 06:18:28PM +1000, Nicholas Piggin wrote:
>> >> Excerpts from peterz@...radead.org's message of August 7, 2020 9:11 pm:
>> >> > 
>> >> > What's wrong with something like this?
>> >> > 
>> >> > AFAICT there's no reason to actually try and add IRQ tracing here, it's
>> >> > just a hand full of instructions at the most.
>> >> 
>> >> Because we may want to use that in other places as well, so it would
>> >> be nice to have tracing.
>> >> 
>> >> Hmm... also, I thought NMI context was free to call local_irq_save/restore
>> >> anyway so the bug would still be there in those cases?
>> > 
>> > NMI code has in_nmi() true, in which case the IRQ tracing is disabled
>> > (except for x86 which has CONFIG_TRACE_IRQFLAGS_NMI).
>> > 
>> 
>> That doesn't help. It doesn't fix the lockdep irq state going out of
>> synch with the actual irq state. The code which triggered this with the
>> special powerpc irq disable has in_nmi() true as well.
> 
> Urgh, you're talking about using lockdep_assert_irqs*() from NMI
> context?
> 
> If not, I'm afraid I might've lost the plot a little on what exact
> failure case we're talking about.
> 

Hm, I may have been a bit confused actually. Since your Fix 
TRACE_IRQFLAGS vs NMIs patch it might now work.

I'm worried powerpc disables trace irqs trace_hardirqs_off()
before nmi_enter() might still be a problem, but not sure
actually. Alexey did you end up re-testing with Peter's patch
or current upstream?

Thanks,
Nick