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Date:   Wed, 11 Jul 2018 08:15:59 -0700
From:   "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        linux-kernel@vger.kernel.org, Boqun Feng <boqun.feng@gmail.com>,
        Byungchul Park <byungchul.park@lge.com>,
        Ingo Molnar <mingo@redhat.com>,
        Julia Cartwright <julia@ni.com>,
        linux-kselftest@vger.kernel.org,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Thomas Glexiner <tglx@linutronix.de>,
        Tom Zanussi <tom.zanussi@linux.intel.com>
Subject: Re: [PATCH v9 4/7] tracepoint: Make rcuidle tracepoint callers use
 SRCU
Reply-To: paulmck@linux.vnet.ibm.com
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On Wed, Jul 11, 2018 at 10:46:18AM -0400, Steven Rostedt wrote:
> On Wed, 11 Jul 2018 07:27:44 -0700
> "Paul E. McKenney" <paulmck@linux.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@infradead.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.

Indeed, the rcu_dereference_sched() would catch it in that case, so
agreed, Peter's suggestion isn't losing any debuggability.

						Thanx, Paul