Re: [PATCH tip/core/rcu 04/15] rcu: Permit RCU_NONIDLE() to be usedfrom interrupt context

[Josh Triplett]

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@xxxxxxxxxx>
> > > > 
> > > > 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@xxxxxxxxxxx>
> > > > Signed-off-by: Paul E. McKenney <paul.mckenney@xxxxxxxxxx>
> > > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > > 
> > > 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
+
 /*
  * 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.

- Josh Triplett
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