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

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Date:   Tue, 26 Apr 2022 16:17:06 +0800
From:   Mark-PK Tsai <mark-pk.tsai@...iatek.com>
To:     <rostedt@...dmis.org>
CC:     <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-mediatek@...ts.infradead.org>, <mark-pk.tsai@...iatek.com>,
        <matthias.bgg@...il.com>, <mingo@...hat.com>,
        <yj.chiang@...iatek.com>
Subject: Re: [PATCH v3 1/2] tracing: Avoid adding tracer option before update_tracer_options

> > 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.

Calling __update_tracer_options() when tracer_options_updated=false have no
effect because it return directly in add_tracer_options().
I will fix it in v4 as you suggest in the below comment.

> 
> > ---
> >  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.

You are right.
I will update in v4, thanks!

> 
> -- Steve
> 
> 
> >  	mutex_unlock(&trace_types_lock);
> >  }
> >