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

From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Ingo Molnar <mingo@kernel.org>,
	"Joel Fernandes, Google" <joel@joelfernandes.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Gustavo A. R. Silva" <gustavo@embeddedor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	paulmck <paulmck@kernel.org>,
	Josh Triplett <josh@joshtriplett.org>,
	Lai Jiangshan <jiangshanlai@gmail.com>
Subject: Re: [PATCH v2] tracing/perf: Move rcu_irq_enter/exit_irqson() to perf trace point hook
Date: Tue, 11 Feb 2020 11:27:36 -0500 (EST)	[thread overview]
Message-ID: <698566505.617724.1581438456170.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <20200211111828.48058768@gandalf.local.home>

----- On Feb 11, 2020, at 11:18 AM, rostedt rostedt@goodmis.org wrote:

> On Tue, 11 Feb 2020 16:34:52 +0100
> Peter Zijlstra <peterz@infradead.org> 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