Message-ID: <20120906185402.GB5738@jtriplet-mobl1>
Date:	Thu, 6 Sep 2012 11:54:02 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, fweisbec@...il.com,
	sbw@....edu, patches@...aro.org,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH tip/core/rcu 04/15] rcu: Permit RCU_NONIDLE() to be used
 from interrupt context

On Tue, Sep 04, 2012 at 04:43:07PM -0700, Paul E. McKenney wrote:
> On Tue, Sep 04, 2012 at 04:33:44PM -0700, Josh Triplett wrote:
> > On Tue, Sep 04, 2012 at 07:23:51PM -0400, Steven Rostedt wrote:
> > > On Tue, 2012-09-04 at 16:08 -0700, Josh Triplett wrote:
> > > > On Tue, Sep 04, 2012 at 06:51:22PM -0400, Steven Rostedt wrote:
> > > > > On Tue, 2012-09-04 at 15:33 -0700, Paul E. McKenney wrote:
> > > > > > On Fri, Aug 31, 2012 at 11:00:52AM -0700, Josh Triplett wrote:
> > > > > > > On Thu, Aug 30, 2012 at 11:56:17AM -0700, Paul E. McKenney wrote:
> > > > > > > > From: "Paul E. McKenney" <paul.mckenney@...aro.org>
> > > > > > > > 
> > > > > > > > There is a need to use RCU from interrupt context, but either before
> > > > > > > > rcu_irq_enter() is called or after rcu_irq_exit() is called.  If the
> > > > > > > > interrupt occurs from idle, then lockdep-RCU will complain about such
> > > > > > > > uses, as they appear to be illegal uses of RCU from the idle loop.
> > > > > > > > In other environments, RCU_NONIDLE() could be used to properly protect
> > > > > > > > the use of RCU, but RCU_NONIDLE() currently cannot be invoked except
> > > > > > > > from process context.
> > > > > > > > 
> > > > > > > > This commit therefore modifies RCU_NONIDLE() to permit its use more
> > > > > > > > globally.
> > > > > > > > 
> > > > > > > > Reported-by: Steven Rostedt <rostedt@...dmis.org>
> > > > > > > > Signed-off-by: Paul E. McKenney <paul.mckenney@...aro.org>
> > > > > > > > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > > > > > > 
> > > > > > > Something seems wrong about this.  The addition of EXPORT_SYMBOL_GPL
> > > > > > > suggests that such interrupt handlers might live in modules.  In what
> > > > > > > situation might a module interrupt handler get called from the idle
> > > > > > > loop, before rcu_irq_enter or after rcu_irq_exit, and need to know that
> > > > > > > when using RCU?
> > > > > > 
> > > > > > Drivers can be in modules, in which case their interrupt handlers will
> > > > > > also be in the corresponding module.  I do agree that in most cases,
> > > > > > the irq_enter() and irq_exit() hooks would be invoked by non-module code,
> > > > > > but I do believe that I had to add those exports due to build failures.
> > > > > > 
> > > > > > Steven will let me know if I am confused on this point.
> > > > > > 
> > > > > 
> > > > > You're not confused, the situation is confusing :-/
> > > > > 
> > > > > Because some trace events happen inside the idle loop after rcu has
> > > > > "shutdown", we needed to create "trace_foo_rcuidle()" handlers that can
> > > > > handle this condition. That is, for every trace_foo() static inline
> > > > > (used at the tracepoint location), there exists a static inline
> > > > > trace_foo_rcuidle(), that looks something like this:
> > > > > 
> > > > > static inline void trace_##name##_rcuidle(proto) {
> > > > > 	if (static_key_false(&__tracepoint_##name.key)) { 
> > > > > 		rcu_idle_exit();
> > > > > 		__DO_TRACE();
> > > > > 		rcu_idle_enter();
> > > > > 	}
> > > > > }
> > > > > 
> > > > > Although these calls are never used by module code, because they are
> > > > > static inlines, they are still defined for all tracepoints, kernel
> > > > > tracepoints as well as module tracepoints. And thus, need the export :-(
> > > > 
> > > > Fair enough.
> > > > 
> > > > What about having the tracepoint code generation detect when building as
> > > > part of a module via defined(MODULE), and omit the unused _rcuidle
> > > > versions in those cases?  That would avoid the need to export those
> > > > functions at all.  Strawman patch (not tested):
> > > > 
> > > > diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> > > > index 802de56..41e1ef2 100644
> > > > --- a/include/linux/tracepoint.h
> > > > +++ b/include/linux/tracepoint.h
> > > > @@ -136,6 +136,22 @@ static inline void tracepoint_synchronize_unregister(void)
> > > >  		postrcu;						\
> > > >  	} while (0)
> > > >  
> > > > +#ifdef 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),			\
> > > > +				rcu_idle_exit(),			\
> > > > +				rcu_idle_enter());			\
> > > > +	}
> > > > +#else
> > > > +#define __DECLARE_TRACE_RCU(name, proto, args, cond, data_proto, data_args)
> > > > +#endif
> > > > +
> > > 
> > > Egad! More macros on top of macros! Yeah I know I'm the most guilty of
> > > this, but it just seems to add one more indirection that I would like to
> > > avoid.
> > 
> > This doesn't seem to add a significant amount of complexity; it forwards
> > exactly the same parameters to the helper macro, and just moves the one
> > function definition there and makes it conditional.  This still seems
> > more preferable than exporting functions just so they won't get called.
> > 
> > > >  /*
> > > >   * Make sure the alignment of the structure in the __tracepoints section will
> > > >   * not add unwanted padding between the beginning of the section and the
> > > > @@ -151,16 +167,7 @@ static inline void tracepoint_synchronize_unregister(void)
> > > >  				TP_ARGS(data_args),			\
> > > >  				TP_CONDITION(cond),,);			\
> > > >  	}								\
> > > > -	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),			\
> > > > -				rcu_idle_exit(),			\
> > > > -				rcu_idle_enter());			\
> > > > -	}								\
> > > > +	__DECLARE_TRACE_RCU(name, proto, args, cond, data_proto, data_args) \
> > > >  	static inline int						\
> > > >  	register_trace_##name(void (*probe)(data_proto), void *data)	\
> > > >  	{								\
> > > > 
> > > > 
> > > > If that doesn't work out, please consider adding an explicit comment
> > > > saying why you exported the functions.
> > > > 
> > > 
> > > Or, we could also add in include/linux/rcupdate.h:
> > > 
> > > #ifdef MODULE
> > > static inline void rcu_idle_enter(void) {
> > > 	panic("Don't call me from modules");
> > > }
> > > static inline void rcu_idle_exit(void) {
> > > 	panic("Don't call me from modules");
> > > }
> > > #else
> > > extern void rcu_idle_enter(void);
> > > extern void rcu_idle_exit(void);
> > > #endif
> > 
> > I could live with that; it seems preferable to unnecessary exports,
> > though it still seems much uglier to me than the conditional definition
> > of trace_*_rcuidle. :)
> 
> Not sure I see much difference in aesthetics between the three approaches,
> but am willing to switch over to a generally agreed-upon scheme.

Steve, could I get an ack from you on the patch I sent?

- Josh Triplett
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