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

[Nicholas Piggin <npiggin@gmail.com>]

Excerpts from peterz@infradead.org's message of July 26, 2020 10:11 pm:
> On Sun, Jul 26, 2020 at 02:14:34PM +1000, Nicholas Piggin wrote:
>> Excerpts from Peter Zijlstra's message of July 26, 2020 6:26 am:
> 
>> > Which is 'funny' when it interleaves like:
>> > 
>> > 	local_irq_disable();
>> > 	...
>> > 	local_irq_enable()
>> > 	  trace_hardirqs_on();
>> > 	  <NMI/>
>> > 	  raw_local_irq_enable();
>> > 
>> > Because then it will undo the trace_hardirqs_on() we just did. With the
>> > result that both tracing and lockdep will see a hardirqs-disable without
>> > a matching enable, while the hardware state is enabled.
>> 
>> Seems like an arch problem -- why not disable if it was enabled only?
>> I guess the local_irq tracing calls are a mess so maybe they copied 
>> those.
> 
> Because, as I wrote earlier, then we can miss updating software state.
> So your proposal has:
> 
> 	raw_local_irq_disable()
> 	<NMI>
> 	  if (!arch_irqs_disabled(regs->flags) // false
> 	    trace_hardirqs_off();
> 
> 	  // tracing/lockdep still think IRQs are enabled
> 	  // hardware IRQ state is disabled.

... and then lockdep_nmi_enter can disable IRQs if they were enabled?

The only reason it's done this way as opposed to a much simple counter 
increment/decrement AFAIKS is to avoid some overhead of calling 
trace_hardirqs_on/off (which seems a bit dubious but let's go with it).

In that case the lockdep_nmi_enter code is the right spot to clean up 
that gap vs NMIs. I guess there's an argument that arch_nmi_enter could
do it. I don't see the problem with fixing it up here though, this is a 
slow path so it doesn't matter if we have some more logic for it.

Thanks,
Nick