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

[Peter Zijlstra]

On Mon, Feb 10, 2020 at 05:06:43PM -0500, Steven Rostedt wrote:
> +	if (!rcu_watching) {						\
> +		/* Can not use RCU if rcu is not watching and in NMI */	\
> +		if (in_nmi())						\
> +			return;						\
> +		rcu_irq_enter_irqson();					\
> +	}								\

I saw the same weirdness in __trace_stack(), and I'm confused by it.

How can we ever get to: in_nmi() && !rcu_watching() ? That should be a
BUG.  In particular, nmi_enter() has rcu_nmi_enter().

Paul, can that really happen?