linux-kernel - Re: [RFC][PATCH 19/18] ftrace: Print a message when the rcu checker is disabled

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Date:	Tue, 3 Sep 2013 18:25:05 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jiri Olsa <jolsa@...hat.com>
Subject: Re: [RFC][PATCH 19/18] ftrace: Print a message when the rcu checker
 is disabled

On Tue, Sep 03, 2013 at 09:11:27PM -0400, Steven Rostedt wrote:
> From f8f5d278e272c42349b3cd32485faf426d0c459e Mon Sep 17 00:00:00 2001
> From: "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>
> Date: Tue, 3 Sep 2013 20:47:59 -0400
> Subject: [PATCH] ftrace: Print a message when the rcu checker is disabled
> 
> Let the user know that the RCU safety checker for function tracing
> has been disabled. The checker only runs when the user specifically
> configures it in the kernel, and this is done to search for locations
> in the kernel that may be unsafe for a function trace callback to
> use rcu_read_lock()s. But if things like function graph tracing is
> started, which can live lock the machine when the checker is running,
> it is disabled. It would be nice if the kernel let the user know that
> the checker is disabled, and when it is re-enabled again.
> 
> As the checker only gets disabled and re-enabled by user actions
> (starting and stoping the function graph tracer or the irqsoff tracer)
> it is fine to have a printk display that it is disabled or not.
> 
> Suggested-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>

Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

> ---
>  kernel/trace/trace_functions.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/trace/trace_functions.c b/kernel/trace/trace_functions.c
> index 0883069..dd59827 100644
> --- a/kernel/trace/trace_functions.c
> +++ b/kernel/trace/trace_functions.c
> @@ -569,6 +569,8 @@ void ftrace_unsafe_rcu_checker_disable(void)
>  	atomic_inc(&ftrace_unsafe_rcu_disabled);
>  	/* Make sure the update is seen immediately */
>  	smp_wmb();
> +	pr_info("Disabled FTRACE RCU checker (%pS)\n",
> +		__builtin_return_address(0));
>  }
>  
>  void ftrace_unsafe_rcu_checker_enable(void)
> @@ -576,6 +578,8 @@ void ftrace_unsafe_rcu_checker_enable(void)
>  	atomic_dec(&ftrace_unsafe_rcu_disabled);
>  	/* Make sure the update is seen immediately */
>  	smp_wmb();
> +	pr_info("Enabled FTRACE RCU checker (%pS)\n",
> +		__builtin_return_address(0));
>  }
>  
>  static DEFINE_PER_CPU(unsigned long, ftrace_rcu_func);
> -- 
> 1.8.1.4
> 

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