Re: [PATCH 7/7 v3] tracing: Make sure rcu_irq_enter() can work for trace_*_rcuidle() trace events

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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
> 
>