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SubjectRe: [PATCH 1/2] lockdep: improve current->(hard|soft)irqs_enabled synchronisation with actual irq state
Excerpts from Peter Zijlstra's message of July 23, 2020 9:40 pm:
> On Thu, Jul 23, 2020 at 08:56:14PM +1000, Nicholas Piggin wrote:
>
>> diff --git a/arch/powerpc/include/asm/hw_irq.h b/arch/powerpc/include/asm/hw_irq.h
>> index 3a0db7b0b46e..35060be09073 100644
>> --- a/arch/powerpc/include/asm/hw_irq.h
>> +++ b/arch/powerpc/include/asm/hw_irq.h
>> @@ -200,17 +200,14 @@ static inline bool arch_irqs_disabled(void)
>> #define powerpc_local_irq_pmu_save(flags) \
>> do { \
>> raw_local_irq_pmu_save(flags); \
>> - trace_hardirqs_off(); \
>> + if (!raw_irqs_disabled_flags(flags)) \
>> + trace_hardirqs_off(); \
>> } while(0)
>> #define powerpc_local_irq_pmu_restore(flags) \
>> do { \
>> - if (raw_irqs_disabled_flags(flags)) { \
>> - raw_local_irq_pmu_restore(flags); \
>> - trace_hardirqs_off(); \
>> - } else { \
>> + if (!raw_irqs_disabled_flags(flags)) \
>> trace_hardirqs_on(); \
>> - raw_local_irq_pmu_restore(flags); \
>> - } \
>> + raw_local_irq_pmu_restore(flags); \
>> } while(0)
>
> You shouldn't be calling lockdep from NMI context!

After this patch it doesn't.

trace_hardirqs_on/off implementation appears to expect to be called in NMI
context though, for some reason.

> That is, I recently
> added suport for that on x86:
>
> https://lkml.kernel.org/r/20200623083721.155449112@infradead.org
> https://lkml.kernel.org/r/20200623083721.216740948@infradead.org
>
> But you need to be very careful on how you order things, as you can see
> the above relies on preempt_count() already having been incremented with
> NMI_MASK.

Hmm. My patch seems simpler.

I don't know this stuff very well, I don't really understand what your patch
enables for x86 but at least it shouldn't be incompatible with this one
AFAIKS.

Thanks,
Nick

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