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

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Date:   Fri, 24 Jul 2020 02:20:20 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Peter Zijlstra <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 Peter Zijlstra's message of July 24, 2020 12:59 am:
> On Thu, Jul 23, 2020 at 11:11:03PM +1000, Nicholas Piggin wrote:
>> 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.
> 
> You sure, trace_hardirqs_{on,off}() calls into lockdep.

At least for irq enable/disable functions yes. NMI runs with
interrupts disabled so these will never call trace_hardirqs_on/off
after this patch.

> (FWIW they're
> also broken vs entry ordering, but that's another story).
> 
>> trace_hardirqs_on/off implementation appears to expect to be called in NMI 
>> context though, for some reason.
> 
> Hurpm, not sure.. I'll have to go grep arch code now :/ The generic NMI
> code didn't touch that stuff.
> 
> Argh, yes, there might be broken there... damn! I'll go frob around.
> 

Thanks,
Nick