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Date: Tue, 4 Sep 2012 16:43:07 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Josh Triplett <josh@...htriplett.org>
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 04:33:44PM -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.
Thanx, Paul
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