linux-kernel - Re: [PATCH v9 4/7] tracepoint: Make rcuidle tracepoint callers use SRCU

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Date:   Wed, 11 Jul 2018 23:21:20 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Boqun Feng <boqun.feng@...il.com>,
        Byungchul Park <byungchul.park@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Julia Cartwright <julia@...com>,
        linux-kselftest@...r.kernel.org,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Thomas Glexiner <tglx@...utronix.de>,
        Tom Zanussi <tom.zanussi@...ux.intel.com>
Subject: Re: [PATCH v9 4/7] tracepoint: Make rcuidle tracepoint callers use
 SRCU

On Wed, 11 Jul 2018 13:52:49 -0700
Joel Fernandes <joel@...lfernandes.org> wrote:

> > #define __DECLARE_TRACE(name, proto, args, cond, data_proto, data_args) \
> > 	extern struct tracepoint __tracepoint_##name;			\
> > 	static inline void trace_##name(proto)				\
> > 	{								\
> > 		if (static_key_false(&__tracepoint_##name.key))		\
> > 			__DO_TRACE(&__tracepoint_##name,		\
> > 				TP_PROTO(data_proto),			\
> > 				TP_ARGS(data_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();		\
> > 		}							\
> > 	}
> > 
> > Because lockdep would only trigger warnings when the tracepoint was
> > enabled and used in a place it shouldn't be, we added the above
> > IS_ENABLED(CONFIG_LOCKDEP) part to test regardless if the the
> > tracepoint was enabled or not. Because we do this, we don't need to
> > have the test in the __DO_TRACE() code itself. That means we can clean
> > up the code as per Peter's suggestion.  
> 
> Sounds good, I'm Ok with making this change.
> 
> Just to clarify, are you proposing to change the rcu_dereference_sched to
> rcu_dereference_raw in both __DECLARE_TRACE and __DO_TRACE?

No, just in __DO_TRACE(). The rcu_dereference_sched() above in
__DECLARE_TRACE() in the if (IS_ENABLED(CONFIG_LOCKDEP) block is
required to show the warnings if trace_##name() is used wrong, and is
the reason we can use rcu_dereference_raw() in __DO_TRACE() in the
first place ;-)

This brings up another point. We should probably add to
__DECLARE_TRACE_RCU() this:

#ifndef MODULE
#define __DECLARE_TRACE_RCU(name, proto, args, cond, data_proto, data_args) \
	static inline void trace_##name##_rcuidle(proto)		\
	{								\
		if (static_key_false(&__tracepoint_##name.key))		\
			__DO_TRACE(&__tracepoint_##name,		\
				TP_PROTO(data_proto),			\
				TP_ARGS(data_args),			\
				TP_CONDITION(cond), 1);			\
+		if (IS_ENABLED(CONFIG_LOCKDEP) && (cond)) {		\
+			int idx;					\
+			idx = srcu_read_lock_notrace(&tracepoint_srcu); \
+			srcu_dereference_notrace(__tracepoint_##name.funcs, \
+						&tracepoint_srcu);	\
+			srcu_read_unlock_notrace(&tracepoint_srcu, idx); \
+		}							\
	}
#else


So that lockdep works with trace_##name##__rcuidle() when the trace
event is not enabled.

But that should be a separate patch and not part of this series. I may
write that up tomorrow.

-- Steve