Re: [PATCH v3] tracing: Make sure RCU is watching before calling a stack trace

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

On Thu, May 18, 2017 at 09:11:40PM -0400, Steven Rostedt wrote:
> From 8260b0308d3ee0f836cd5ea4b177efeb52ab151f Mon Sep 17 00:00:00 2001
> From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> Date: Fri, 12 May 2017 13:15:45 -0400
> Subject: [PATCH] tracing: Make sure RCU is watching before calling a stack
>  trace
> 
> As stack tracing now requires "rcu watching", force RCU to be watching when
> recording a stack trace.
> 
> Link: http://lkml.kernel.org/r/20170512172449.879684501@goodmis.org
> 
> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

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

> ---
> 
> Changes since v2:
> 
>  Use rcu_is_watching() instead if checking for in_nmi() and irqs
>  disabled. But now if rcu_is_watching() fails and in_nmi() is true,
>  bail.
> 
>  kernel/trace/trace.c | 31 ++++++++++++++++++++++++++++++-
>  1 file changed, 30 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index fcc9a2d..1122f15 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -2568,7 +2568,36 @@ static inline void ftrace_trace_stack(struct trace_array *tr,
>  void __trace_stack(struct trace_array *tr, unsigned long flags, int skip,
>  		   int pc)
>  {
> -	__ftrace_trace_stack(tr->trace_buffer.buffer, flags, skip, pc, NULL);
> +	struct ring_buffer *buffer = tr->trace_buffer.buffer;
> +
> +	if (rcu_is_watching()) {
> +		__ftrace_trace_stack(buffer, flags, skip, pc, NULL);
> +		return;
> +	}
> +
> +	/*
> +	 * When an NMI triggers, RCU is enabled via rcu_nmi_enter(),
> +	 * but if the above rcu_is_watching() failed, then the NMI
> +	 * triggered someplace critical, and rcu_irq_enter() should
> +	 * not be called from NMI.
> +	 */
> +	if (unlikely(in_nmi()))
> +		return;
> +
> +	/*
> +	 * It is possible that a function is being traced in a
> +	 * location that RCU is not watching. A call to
> +	 * rcu_irq_enter() will make sure that it is, but there's
> +	 * a few internal rcu functions that could be traced
> +	 * where that wont work either. In those cases, we just
> +	 * do nothing.
> +	 */
> +	if (unlikely(rcu_irq_enter_disabled()))
> +		return;
> +
> +	rcu_irq_enter_irqson();
> +	__ftrace_trace_stack(buffer, flags, skip, pc, NULL);
> +	rcu_irq_exit_irqson();
>  }
>  
>  /**
> -- 
> 2.9.3
>