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

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Date:   Wed, 11 Jul 2018 10:46:18 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Joel Fernandes <joel@...lfernandes.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 07:27:44 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> On Wed, Jul 11, 2018 at 09:00:03AM -0400, Steven Rostedt wrote:
> > On Wed, 11 Jul 2018 14:49:54 +0200
> > Peter Zijlstra <peterz@...radead.org> wrote:
> >   
> > > On Thu, Jun 28, 2018 at 11:21:46AM -0700, Joel Fernandes wrote:  
> > > > -		it_func_ptr = rcu_dereference_sched((tp)->funcs);	\    
> > > 
> > > I would convert to rcu_dereference_raw() to appease sparse. The fancy
> > > stuff below is pointless if you then turn off all checking.  
> > 
> > The problem with doing this is if we use a trace event without the
> > proper _idle() or whatever, we wont get a warning that it is used
> > incorrectly with lockdep. Or does lockdep still check if "rcu is
> > watching" with rcu_dereference_raw()?  
> 
> No lockdep checking is done by rcu_dereference_raw().

Correct, but I think we can do this regardless. So Joel please resend
with Peter's suggestion.

The reason being is because of this:


#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.

-- Steve