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

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Message-ID: <20180712191701.GA107829@joelaf.mtv.corp.google.com>
Date:   Thu, 12 Jul 2018 12:17:01 -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 Thu, Jul 12, 2018 at 09:35:12AM -0400, Steven Rostedt wrote:

> On Wed, 11 Jul 2018 21:28:25 -0700
> Joel Fernandes <joel@...lfernandes.org> wrote:
> 
> > On Wed, Jul 11, 2018 at 11:21:20PM -0400, Steven Rostedt wrote:
> > > 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.  
> > 
> > Yes, that sounds good to me and would be good to add the safe guard there.
> > But you meant srcu_dereference above, not srcu_dereference_notrace right?
> 
> We don't need to trace them. I believe that the "srcu_*_notrace" still
> performs the lockdep checks. That's what we want. If they don't then we
> should not use notrace. But I believe they still do lockdep.

AFAICT, _notrace doesn't call into lockdep or tracing (there's also a comment
that says so):

/**
 * srcu_dereference_notrace - no tracing and no lockdep calls from here
 */

So then, we should use the regular variant for this additional check you're
suggesting.

thanks,

- Joel