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SubjectRe: [PATCH 7/7 v3] tracing: Make sure rcu_irq_enter() can work for trace_*_rcuidle() trace events
On Mon, Apr 10, 2017 at 02:10:12PM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
>
> Stack tracing discovered that there's a small location inside the RCU
> infrastructure where calling rcu_irq_enter() does not work. As trace events
> use rcu_irq_enter() it must make sure that it is functionable. A check
> against rcu_irq_enter_disabled() is added with a WARN_ON_ONCE() as no trace
> event should ever be used in that part of RCU. If the warning is triggered,
> then the trace event is ignored.
>
> Restructure the __DO_TRACE() a bit to get rid of the prercu and postrcu,
> and just have an rcucheck that does the work from within the _DO_TRACE()
> macro. gcc optimization will compile out the rcucheck=0 case.
>
> Link: http://lkml.kernel.org/r/20170405093207.404f8deb@gandalf.local.home
>
> Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

As an aside, it looks like the rcu_irq_enter_disabled() settings
in the RCU idle-entry code could be placed under CONFIG_PROVE_RCU=y
should my idle-entry-overhead concerns prove to be well-founded.
The errors would be caught during testing, but no production-side
overhead.

Again, I am not necessarily agitating for this change now, just getting
this possibility on the record. ;-)

Thanx, Paul

> ---
> include/linux/tracepoint.h | 19 +++++++++++--------
> 1 file changed, 11 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> index f72fcfe0e66a..cc48cb2ce209 100644
> --- a/include/linux/tracepoint.h
> +++ b/include/linux/tracepoint.h
> @@ -128,7 +128,7 @@ extern void syscall_unregfunc(void);
> * as "(void *, void)". The DECLARE_TRACE_NOARGS() will pass in just
> * "void *data", where as the DECLARE_TRACE() will pass in "void *data, proto".
> */
> -#define __DO_TRACE(tp, proto, args, cond, prercu, postrcu) \
> +#define __DO_TRACE(tp, proto, args, cond, rcucheck) \
> do { \
> struct tracepoint_func *it_func_ptr; \
> void *it_func; \
> @@ -136,7 +136,11 @@ extern void syscall_unregfunc(void);
> \
> if (!(cond)) \
> return; \
> - prercu; \
> + if (rcucheck) { \
> + if (WARN_ON_ONCE(rcu_irq_enter_disabled())) \
> + return; \
> + rcu_irq_enter_irqson(); \
> + } \
> rcu_read_lock_sched_notrace(); \
> it_func_ptr = rcu_dereference_sched((tp)->funcs); \
> if (it_func_ptr) { \
> @@ -147,20 +151,19 @@ extern void syscall_unregfunc(void);
> } while ((++it_func_ptr)->func); \
> } \
> rcu_read_unlock_sched_notrace(); \
> - postrcu; \
> + if (rcucheck) \
> + rcu_irq_exit_irqson(); \
> } while (0)
>
> #ifndef MODULE
> -#define __DECLARE_TRACE_RCU(name, proto, args, cond, data_proto, data_args) \
> +#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_irq_enter_irqson(), \
> - rcu_irq_exit_irqson()); \
> + TP_CONDITION(cond), 1); \
> }
> #else
> #define __DECLARE_TRACE_RCU(name, proto, args, cond, data_proto, data_args)
> @@ -186,7 +189,7 @@ extern void syscall_unregfunc(void);
> __DO_TRACE(&__tracepoint_##name, \
> TP_PROTO(data_proto), \
> TP_ARGS(data_args), \
> - TP_CONDITION(cond),,); \
> + TP_CONDITION(cond), 0); \
> if (IS_ENABLED(CONFIG_LOCKDEP) && (cond)) { \
> rcu_read_lock_sched_notrace(); \
> rcu_dereference_sched(__tracepoint_##name.funcs);\
> --
> 2.10.2
>
>

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