Re: [PATCH] tracing, hardirq: Do not test lockdep on irq trace points when disabled

[Peter Zijlstra]

On Tue, Mar 07, 2023 at 06:46:45PM -0500, Steven Rostedt wrote:
> From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>
> 
> When CONFIG_LOCKDEP is enabled, the trace points have:
> 
> 	static inline void trace_##name(proto)				\
> 	{								\
> 		if (static_key_false(&__tracepoint_##name.key))		\
> 			__DO_TRACE(name,				\
> 				TP_ARGS(args),				\
> 				TP_CONDITION(cond), 0);			\
> 		if (IS_ENABLED(CONFIG_LOCKDEP) && (cond)) {		\
> 			rcu_read_lock_sched_notrace();			\
> 			rcu_dereference_sched(__tracepoint_##name.funcs);\
> 			rcu_read_unlock_sched_notrace();		\
> 		}							\
> 	}								\

> Where it will test lockdep for trace points even when they are not
> enabled, to make sure they do not cause RCU issues, and lockdep will
> trigger even when the trace points are not enabled.

I'm confused what that's actually trying to do..

You're not tickling the rcu_is_watching() check, because
rcu_read_lock_sched_notrace() doesn't have that. You're not tickling
RCU_LOCKDEP_WARN() because you did rcu_read_lock_sched_notrace().

So what?!?