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 17:19:05 +0000 (UTC)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     rostedt <rostedt@...dmis.org>
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 Apr 7, 2017, at 1:06 PM, rostedt rostedt@...dmis.org wrote:

> From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>
> 
> Stack tracing discovered that there's a small location inside the RCU
> infrastructure that calling rcu_irq_enter() does not work. As trace events

that -> where

Do you have a link to the lkml thread where this stack tracing discovery
happened ?

> use rcu_irq_enter() it must make sure that it is functionable. A check

I don't think functionable is the word you are looking for here. Perhaps
"must make sure that it can be invoked" ?

> 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.
> 
> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> ---
> include/linux/tracepoint.h | 2 ++
> 1 file changed, 2 insertions(+)
> 
> diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> index f72fcfe..8baef96 100644
> --- a/include/linux/tracepoint.h
> +++ b/include/linux/tracepoint.h
> @@ -159,6 +159,8 @@ extern void syscall_unregfunc(void);
> 				TP_PROTO(data_proto),			\
> 				TP_ARGS(data_args),			\
> 				TP_CONDITION(cond),			\
> +				if (WARN_ON_ONCE(rcu_irq_enter_disabled())) \
> +					return;				\

I must admit that it's a bit odd to have:

if (WARN_ON_ONCE(rcu_irq_enter_disabled()))
       return;
rcu_irq_enter_irqson()

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 ?

Thanks,

Mathieu


> 				rcu_irq_enter_irqson(),			\
> 				rcu_irq_exit_irqson());			\
> 	}
> --
> 2.9.3

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com