linux-kernel - Re: [PATCH v3 1/2] tracing: Avoid adding tracer option before update_tracer

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Message-ID: <20220422174258.1f41777a@gandalf.local.home>
Date:   Fri, 22 Apr 2022 17:42:58 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mark-PK Tsai <mark-pk.tsai@...iatek.com>
Cc:     <mingo@...hat.com>, <matthias.bgg@...il.com>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-mediatek@...ts.infradead.org>, <yj.chiang@...iatek.com>
Subject: Re: [PATCH v3 1/2] tracing: Avoid adding tracer option before
 update_tracer_options

On Wed, 23 Mar 2022 23:22:56 +0800
Mark-PK Tsai <mark-pk.tsai@...iatek.com> wrote:

> To prepare for support asynchronous tracer_init_tracefs initcall,
> avoid calling create_trace_option_files before __update_tracer_options.
> Otherwise, create_trace_option_files will show warning because
> some tracers in trace_types list are already in tr->topts.
> 
> For example, hwlat_tracer call register_tracer in late_initcall,
> and global_trace.dir is already created in tracing_init_dentry,
> hwlat_tracer will be put into tr->topts.
> Then if the __update_tracer_options is executed after hwlat_tracer
> registered, create_trace_option_files find that hwlat_tracer is
> already in tr->topts.
> 
> 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>

Before this patch:

 # ls /sys/kernel/tracing/options
annotate	   funcgraph-duration  hex		stacktrace
bin		   funcgraph-irqs      irq-info		sym-addr
blk_cgname	   funcgraph-overhead  latency-format	sym-offset
blk_cgroup	   funcgraph-overrun   markers		sym-userobj
blk_classic	   funcgraph-proc      overwrite	test_nop_accept
block		   funcgraph-tail      pause-on-trace	test_nop_refuse
context-info	   func-no-repeats     printk-msg-only	trace_printk
disable_on_free    func_stack_trace    print-parent	userstacktrace
display-graph	   function-fork       raw		verbose
event-fork	   function-trace      record-cmd
funcgraph-abstime  graph-time	       record-tgid
funcgraph-cpu	   hash-ptr	       sleep-time


After this patch:

 # ls /sys/kernel/tracing/options
annotate      disable_on_free  irq-info         raw          trace_printk
bin           display-graph    latency-format   record-cmd   userstacktrace
blk_cgname    event-fork       markers          record-tgid  verbose
blk_cgroup    function-fork    overwrite        stacktrace
blk_classic   function-trace   pause-on-trace   sym-addr
block         hash-ptr         printk-msg-only  sym-offset
context-info  hex              print-parent     sym-userobj

Not good.

> ---
>  kernel/trace/trace.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index eb44418574f9..85ec758c4455 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);
>  	__update_tracer_options(tr);
> +	tracer_options_updated = true;

I think you want to set this before the __update, as doing it after just
makes the update a nop.

-- Steve


>  	mutex_unlock(&trace_types_lock);
>  }
>