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

[Joel Fernandes <joel@joelfernandes.org>]

On Wed, Jul 11, 2018 at 08:15:59AM -0700, Paul E. McKenney wrote:
> 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.

Hmm, but if we are doing the check later anyway, then why not do it in
__DO_TRACE itself?

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.

thanks!

- Joel