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

On Tue, Mar 07, 2023 at 06:46:45PM -0500, Steven Rostedt wrote:

From: "Steven Rostedt (Google)" <rostedt@goodmis.org>

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?!?