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

From: Peter Zijlstra <[email-masked]>
Date: Thu, 23 Jul 2020 16:59:04 +0200

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. (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.