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

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Date:   Wed, 11 Jul 2018 19:35:25 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Steven Rostedt <rostedt@...dmis.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, Jul 11, 2018 at 09:22:37PM -0400, Steven Rostedt wrote:
> On Wed, 11 Jul 2018 13:56:39 -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.  
> > > 
> > > Indeed, the rcu_dereference_sched() would catch it in that case, so
> > > agreed, Peter's suggestion isn't losing any debuggability.  
> > 
> > Hmm, but if we are doing the check later anyway, then why not do it in
> > __DO_TRACE itself?
> 
> Because __DO_TRACE is only called if the trace event is enabled. If we
> never enable a trace event, we never know if it has a potential of
> doing it wrong. The second part is to trigger the warning immediately
> regardless if the trace event is enabled or not.

I see, thanks for the clarification.

> > 
> > Also I guess we are discussing about changing the rcu_dereference_sched which
> > I think should go into a separate patch since my patch isn't touching how the
> > rcuidle==0 paths use the RCU API. So I think this is an existing issue
> > independent of this series.
> 
> But the code you added made it much more complex to keep the checks as
> is. If we remove the checks then this patch doesn't need to have all
> the if statements, and we can do it the way Peter suggested.

Yes, I agree Peter's suggestion is very clean.

> But sure, go ahead and make a separate patch first that removes the
> checks from __DO_TRACE() first if you want to.

No its ok, no problem, I can just do it in the same patch now that I see the code is much simplified with what Peter is suggesting.

thanks!

- Joel