linux-kernel - Re: [PATCH 7/5] tracing: Make sure rcu_irq_enter() can work for trace_*

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Date:   Fri, 7 Apr 2017 13:55:15 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 7/5] tracing: Make sure rcu_irq_enter() can work for
 trace_*_rcuidle() trace events

On Fri, 7 Apr 2017 17:49:17 +0000 (UTC)
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:

> > Welcome to MACRO MAGIC!

Somebody is not wizardly happy.

> >   
> >> 
> >> as one argument to the __DO_TRACE() macro. To me it's a bit unexpected
> >> coding-style wise. Am I the only one not comfortable with the proposed
> >> syntax ?  
> > 
> > The entire TRACE_EVENT()/__DO_TRACE() is special.
> > 
> > I thought about add yet another parameter, but as it doesn't change
> > much, I figured this was good enough. We could beak it up if you like:
> > 
> > #define RCU_IRQ_ENTER_CHECK \
> >	if (WARN_ON_ONCE(rcu_irq_enter_disabled()) 	\
> >		return;					\
> >	rcu_irq_enter_irqson();
> > 
> > [..]
> >			__DO_TRACE(&__tracepoint_##name,		\
> >				TP_PROTO(data_proto),			\
> >				TP_ARGS(data_args),			\
> >				TP_CONDITION(cond),			\
> >				PARAMS(RCU_IRQ_ENTER_CHECK),		\
> >				rcu_irq_exit_irqson());			\
> > 
> > 
> > Would that make you feel more comfortable?  
> 
> No, it's almost worse and adds still adds a return that apply within __DO_TRACE(),
> but which is passed as an argument (code as macro argument), which I find really
> unsettling.

/me finds it strangely enjoyable to make Mathieu unsettled.

> 
> I would prefer to add a new argument to __DO_TRACE, which we can call
> "checkrcu", e.g.:
> 
> #define __DO_TRACE(tp, proto, args, cond, checkrcu, prercu, postrcu)    \

Grumble. I was trying to avoid making the patch more intrusive. But I
do understand your concern.

>         do {                                                            \
>                 struct tracepoint_func *it_func_ptr;                    \
>                 void *it_func;                                          \
>                 void *__data;                                           \
>                                                                         \
>                 if (!((cond) && (checkrcu)))                            \
>                         return;                                         \
>                 prercu;                                                 \
>                 rcu_read_lock_sched_notrace();                          \
>                 it_func_ptr = rcu_dereference_sched((tp)->funcs);       \
>                 if (it_func_ptr) {                                      \
>                         do {                                            \
>                                 it_func = (it_func_ptr)->func;          \
>                                 __data = (it_func_ptr)->data;           \
>                                 ((void(*)(proto))(it_func))(args);      \
>                         } while ((++it_func_ptr)->func);                \
>                 }                                                       \
>                 rcu_read_unlock_sched_notrace();                        \
>                 postrcu;                                                \
>         } while (0)
> 
> And use it like this:
> 
> #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),                     \
>                                 !WARN_ON_ONCE(rcu_irq_enter_disabled()),\
>                                 rcu_irq_enter_irqson(),                 \
>                                 rcu_irq_exit_irqson());                 \
>         }
> 
> This way we only pass evaluated expression (not code with "return" that
> changes the flow) as arguments to __DO_TRACE, which makes it behave more
> like a "sub-function", which is what we usually expect.

I understand what you are getting at, and I will concede your point.
OK, I'll do it your way, but I still think you take all the fun out of
it. ;-)

-- Steve