Re: [PATCH v2] tracing/perf: Move rcu_irq_enter/exit_irqson() to perf trace point hook

[Mathieu Desnoyers]

----- On Feb 11, 2020, at 11:18 AM, rostedt rostedt@xxxxxxxxxxx wrote:

> On Tue, 11 Feb 2020 16:34:52 +0100
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
>> > +		if (unlikely(in_nmi()))
>> > +			goto out;
>> 
>> unless I'm mistaken, we can simply do rcu_nmi_enter() in this case, and
>> rcu_nmi_exit() on the other end.
>> 
>> > +		rcu_irq_enter_irqson();
> 
> The thing is, I don't think this can ever happen. We've had in the
> tracepoint.h:
> 
>		/* srcu can't be used from NMI */			\
>		WARN_ON_ONCE(rcuidle && in_nmi());			\
> 
> And this has yet to trigger.

But that "rcuidle" state is defined on a per-tracepoint basis, whereas
"!rcu_is_watching()" is a state which depends on the current execution
context. I don't follow how the fact that this WARN_ON_ONCE() never
triggered allows us to infer anything about (!rcu_is_watching() && in_nmi()).

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com