linux-kernel - Re: [PATCH 5/5] tracing: reset iterator in t

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Date:	Tue, 23 Jun 2009 15:19:44 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Wang Liming <liming.wang@...driver.com>
CC:	Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/5] tracing: reset iterator in t_start()

> Another version:
> Since we have saved current (struct tracer *) in m->private in .next, in
> .start, we don't need to call .next to find the one that should be
> printed in 2nd or nth time.
> 

I don't like this for 2 reasons.

1. It's strange that @pos is not incremented in next().

2. 
   t_stop()
     mutex_unlock()
                          unregister_tracer(t)
   t_start()
     mutex_lock()
       t = m->private
       ...
       t = t-next.

  We access t->next though @t was unregistered. This is not
  good, though it does no harm here.

> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index cae34c6..02cdccc 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -2055,8 +2055,6 @@ t_next(struct seq_file *m, void *v, loff_t *pos)
>  {
>         struct tracer *t = m->private;
> 
> -       (*pos)++;
> -
>         if (t)
>                 t = t->next;
> 
> @@ -2068,11 +2066,8 @@ t_next(struct seq_file *m, void *v, loff_t *pos)
>  static void *t_start(struct seq_file *m, loff_t *pos)
>  {
>         struct tracer *t = m->private;
> -       loff_t l = 0;
> 
>         mutex_lock(&trace_types_lock);
> -       for (; t && l < *pos; t = t_next(m, t, &l))
> -               ;
> 
>         return t;
>  }
> 
> 
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