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SubjectRe: [PATCH 1/2] lockdep: improve current->(hard|soft)irqs_enabled synchronisation with actual irq state
Excerpts from peterz@infradead.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@infradead.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@infradead.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

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