Re: [PATCH 5/5] tracing: reset iterator in t_start()

[Wang Liming]

Li Zefan wrote:
> > 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().
Yes, it's strang, but we know that @pos sometimes is not necessary, such in this 
position.

> 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.
OK, it's a realy race problem if we call unregister_tracer.
btw: who realy calls this function? :)

Liming Wang
> > 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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