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

From: Nicholas Piggin <[email-masked]>
Date: Fri, 24 Jul 2020 15:59:35 +1000

Excerpts from Alexey Kardashevskiy's message of July 24, 2020 2:16 pm:
> 
> 
> On 23/07/2020 23:11, 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.
>> 
>> 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/[email-masked]
>>>   https://lkml.kernel.org/r/[email-masked]
>>>
>>> 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.
> 
> And your patches fix my error while Peter's do not:
> 
> 
> IRQs not enabled as expected
> WARNING: CPU: 0 PID: 1377 at /home/aik/p/kernel/kernel/softirq.c:169
> __local_bh_enable_ip+0x118/0x190

I think they would have needed some powerpc bits as well. But I don't
see a reason we can't merge my patches, at least they fix this case and
don't seem to make things worse in any way.

Thanks,
Nick