linux-kernel - Re: [PATCH 1/3] tracing/ftrace: don't insert TRACE

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Message-ID: <alpine.DEB.1.10.0812041752210.32726@gandalf.stny.rr.com>
Date:	Thu, 4 Dec 2008 17:55:55 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
cc:	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] tracing/ftrace: don't insert TRACE_PRINT during
 selftests


On Thu, 4 Dec 2008, Frederic Weisbecker wrote:
> Impact: fix tracer selfstests false results
> 
> After setting a ftrace_printk somewhere in th kernel, I saw the
> Function tracer selftest failing.
> 
> When a selftest occurs, the ring buffer is lurked to see if
> some entries were inserted. But concurrent insertion such as
> ftrace_printk could occured at the same time and could give
> false positive or negative results.
> 
> This patch prevent prevent from TRACE_PRINT entries insertion
> during selftests.
> 
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> ---
>  kernel/trace/trace.c |   13 ++++++++++++-
>  1 files changed, 12 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index ea38652..5dca6ef 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -44,6 +44,14 @@
>  unsigned long __read_mostly	tracing_max_latency = (cycle_t)ULONG_MAX;
>  unsigned long __read_mostly	tracing_thresh;
>  
> +/* We need to change this state when a selftest is running.
> + * A selftest will lurk into the ring-buffer to count the
> + * entries inserted during the selftest although some concurrent
> + * insertions into the ring-buffer such as ftrace_printk could occurred
> + * at the same time, giving false positive or negative results.
> + */

The comment style should be:

/*
 * comment
 * comment
 * comment
 */


> +static atomic_t tracing_selftest_running = ATOMIC_INIT(0);

Do we need that is atomic? Also, it needs to be __read_mostly.

> +
>  /* For tracers that don't implement custom flags */
>  static struct tracer_opt dummy_tracer_opt[] = {
>  	{ }
> @@ -589,6 +597,8 @@ int register_tracer(struct tracer *type)
>  		struct tracer *saved_tracer = current_trace;
>  		struct trace_array *tr = &global_trace;
>  		int i;
> +
> +		atomic_set(&tracing_selftest_running, 1);

Enable this in the mutex lock, and we could make it a normal int.

>  		/*
>  		 * Run a selftest on this tracer.
>  		 * Here we reset the trace buffer, and set the current
> @@ -603,6 +613,7 @@ int register_tracer(struct tracer *type)
>  		/* the test is responsible for initializing and enabling */
>  		pr_info("Testing tracer %s: ", type->name);
>  		ret = type->selftest(type, tr);
> +		atomic_set(&tracing_selftest_running, 0);
>  		/* the test is responsible for resetting too */
>  		current_trace = saved_tracer;
>  		if (ret) {
> @@ -3594,7 +3605,7 @@ int trace_vprintk(unsigned long ip, int depth, const char *fmt, va_list args)
>  	unsigned long flags, irq_flags;
>  	int cpu, len = 0, size, pc;
>  
> -	if (tracing_disabled)
> +	if (tracing_disabled || atomic_read(&tracing_selftest_running))
>  		return 0;
>  
>  	pc = preempt_count();
> -- 

-- Steve

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