linux-kernel - Re: [PATCH] tracing: Avoid adding duplicated tracer options when update_tracer

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Message-ID: <20220323093021.4f541b40@gandalf.local.home>
Date:   Wed, 23 Mar 2022 09:30:21 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mark-PK Tsai <mark-pk.tsai@...iatek.com>
Cc:     <oliver.sang@...el.com>, <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-mediatek@...ts.infradead.org>, <lkp@...el.com>,
        <lkp@...ts.01.org>, <matthias.bgg@...il.com>, <mingo@...hat.com>,
        <yj.chiang@...iatek.com>
Subject: Re: [PATCH] tracing: Avoid adding duplicated tracer options when
 update_tracer_options is running in parallel

On Wed, 23 Mar 2022 11:24:42 +0800
Mark-PK Tsai <mark-pk.tsai@...iatek.com> wrote:

> When update_tracer_options is running in parallel,
> tr->tops might be updated before the trace_types list traversal.
> Let update_tracer_options traverse the trace_types list safely in
> kernel init time and avoid the tr->tops update before it finish.

??? Have you seen a bug here? I'm totally confused by this.

> 
> Link: https://lore.kernel.org/lkml/20220322133339.GA32582@xsang-OptiPlex-9020/
> Reported-by: kernel test robot <oliver.sang@...el.com>
> Signed-off-by: Mark-PK Tsai <mark-pk.tsai@...iatek.com>
> ---
>  kernel/trace/trace.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index adb37e437a05..2974ae056068 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -6317,12 +6317,18 @@ static void tracing_set_nop(struct trace_array *tr)
>  	tr->current_trace = &nop_trace;
>  }
>  
> +static bool tracer_options_updated;
> +
>  static void add_tracer_options(struct trace_array *tr, struct tracer *t)
>  {
>  	/* Only enable if the directory has been created already. */
>  	if (!tr->dir)
>  		return;
>  
> +	/* Only create trace option files after update_tracer_options finish */
> +	if (!tracer_options_updated)
> +		return;
> +
>  	create_trace_option_files(tr, t);
>  }
>  
> @@ -9133,6 +9139,7 @@ static void update_tracer_options(struct trace_array *tr)
>  {
>  	mutex_lock(&trace_types_lock);

How is update_trace_options run in parallel?

There's a mutex that protects it. 

-- Steve


>  	__update_tracer_options(tr);
> +	tracer_options_updated = true;
>  	mutex_unlock(&trace_types_lock);
>  }
>