linux-kernel - Re: [RFC][PATCH 1/5] tracing: Make sure RCU is watching before calling a stack trace

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Date:   Fri, 12 May 2017 11:25:35 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [RFC][PATCH 1/5] tracing: Make sure RCU is watching before
 calling a stack trace

On Fri, May 12, 2017 at 01:15:45PM -0400, Steven Rostedt wrote:
> From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>
> 
> As stack tracing now requires "rcu watching", force RCU to be watching when
> recording a stack trace.
> 
> Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>

Assuming that you never get to __trace_stack() if in an NMI handler,
this looks good to me!

In contrast, if if __trace_stack() ever is called from an NMI handler,
invoking rcu_irq_enter() can be fatal.

							Thanx, Paul

> ---
>  kernel/trace/trace.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index c4536c449021..a4208cebb42b 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -2568,7 +2568,12 @@ static inline void ftrace_trace_stack(struct trace_array *tr,
>  void __trace_stack(struct trace_array *tr, unsigned long flags, int skip,
>  		   int pc)
>  {
> +	if (unlikely(rcu_irq_enter_disabled()))
> +		return;
> +
> +	rcu_irq_enter();
>  	__ftrace_trace_stack(tr->trace_buffer.buffer, flags, skip, pc, NULL);
> +	rcu_irq_exit();
>  }
> 
>  /**
> -- 
> 2.10.2
> 
>