linux-kernel - Re: [PATCH] tracepoints: prevents null probe from being added

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Date:	Wed, 20 Mar 2013 22:45:02 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Keun-O Park <kpark3469@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>, keun-o.park@...driver.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tracepoints: prevents null probe from being added

* Keun-O Park (kpark3469@...il.com) wrote:
> On Thu, Mar 21, 2013 at 8:01 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
> > On Wed, 2013-03-20 at 14:01 -0400, Mathieu Desnoyers wrote:
> >> * Steven Rostedt (rostedt@...dmis.org) wrote:
> >> > On Wed, 2013-03-20 at 12:18 +0900, kpark3469@...il.com wrote:
> >> > > From: Sahara <keun-o.park@...driver.com>
> >> > >
> >> > > Somehow tracepoint_entry_add/remove_probe functions allow a null probe
> >> > > function.
> >> >
> >> > You actually hit this in practice, or is this just something that you
> >> > observe from code review?
> >> >
> >> > >  Especially on getting a null probe in remove function, it seems
> >> > > to be used to remove all probe functions in the entry.
> >> >
> >> > Hmm, that actually sounds like a feature.
> >>
> >> Yep. It's been a long time since I wrote this code, but the removal code
> >> seems to use NULL probe pointer to remove all probes for a given
> >> tracepoint.
> >>
> >> I'd be tempted to just validate non-NULL probe within
> >> tracepoint_entry_add_probe() and let other sites as is, just in case
> >> anyone would be using this feature.
> >>
> >> I cannot say that I have personally used this "remove all" feature much
> >> though.
> >>
> >
> > I agree. I don't see anything wrong in leaving the null probe feature in
> > the removal code. But updating the add code looks like a proper change.
> >
> > -- Steve
> >
> >
> 
> Hello Steve & Mathieu,
> If we want to leave the null probe feature enabled, I think it would
> be better modifying the code like the following for code efficiency.
> 
> @@ -112,7 +112,8 @@ tracepoint_entry_add_probe(struct tracepoint_entry *entry,
>         int nr_probes = 0;
>         struct tracepoint_func *old, *new;
> 
> -       WARN_ON(!probe);
> +       if (WARN_ON(!probe))
> +               return ERR_PTR(-EINVAL);
> 
>         debug_print_probes(entry);
>         old = entry->funcs;
> @@ -152,14 +153,15 @@ tracepoint_entry_remove_probe(struct tracepoint_entry *ent
> 
>         debug_print_probes(entry);
>         /* (N -> M), (N > 1, M >= 0) probes */
> -       for (nr_probes = 0; old[nr_probes].func; nr_probes++) {
> -               if (!probe ||
> -                   (old[nr_probes].func == probe &&
> -                    old[nr_probes].data == data))
> -                       nr_del++;
> +       if (probe) {
> +               for (nr_probes = 0; old[nr_probes].func; nr_probes++) {
> +                       if (old[nr_probes].func == probe &&
> +                            old[nr_probes].data == data)
> +                               nr_del++;
> +               }
>         }
> 
> -       if (nr_probes - nr_del == 0) {
> +       if (!probe || nr_probes - nr_del == 0) {

We might want to do:

if (probe) {
  ...
} else {
  nr_del = nr_probes;
}

if (nr_probes - nr_del == 0) {
   ...
}

rather than:

if (probe) {
  ...
}

if (!probe || nr_probes - nr_del == 0) {
   ...
}

Using nr_del makes the code easier to follow IMHO.

Thanks,

Mathieu

>                 /* N -> 0, (N > 1) */
>                 entry->funcs = NULL;
>                 entry->refcount = 0;
> 
> Because we know handing over the null probe to
> tracepoint_entry_add_probe is not possible,
> we don't have to check if the probe is null or not within for loop. If
> the probe is null, it's just enough to add !probe in
> 'if(nr_probes-nr_del==0)'. And, with additional if-clause wrapping
> for-loop, falling through for-loop can be prevented when probe is
> null.
> 
> @@ -173,8 +172,7 @@ tracepoint_entry_remove_probe(struct tracepoint_entry *entry
>                 if (new == NULL)
>                         return ERR_PTR(-ENOMEM);
>                 for (i = 0; old[i].func; i++)
> -                       if (probe &&
> -                           (old[i].func != probe || old[i].data != data))
> +                       if (old[i].func != probe || old[i].data != data)
>                                 new[j++] = old[i];
>                 new[nr_probes - nr_del].func = NULL;
>                 entry->refcount = nr_probes - nr_del;
> 
> We don't have to check the probe here too. We know probe is always true here.
> Thanks.
> 
> -- Kpark

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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