Re: [PATCH v2 6/6] tracing/dynevent: Adopt guard() and scoped_guard()

Masami Hiramatsu
2025-01-05T08:57:41Z

On Fri, 27 Dec 2024 10:12:18 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Sat, 30 Nov 2024 01:48:40 +0900
> "Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:
>
> > From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> >
> > Use guard() or scoped_guard() in dynamic events for critical sections
> > rather than discrete lock/unlock pairs.
> >
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > ---
> > Changes in v2:
> > - Use scoped_guard() instead of guard() to avoid goto warnings.
>
> I forgot you touched this file, and added a free guard to it which
> conflicts. That said, I have some issues with this patch.
>
> > ---
> > kernel/trace/trace_dynevent.c | 35 ++++++++++++++++-------------------
> > 1 file changed, 16 insertions(+), 19 deletions(-)
> >
> > diff --git a/kernel/trace/trace_dynevent.c b/kernel/trace/trace_dynevent.c
> > index 4376887e0d8a..eb8f669c15e1 100644
> > --- a/kernel/trace/trace_dynevent.c
> > +++ b/kernel/trace/trace_dynevent.c
> > @@ -63,9 +63,8 @@ int dyn_event_register(struct dyn_event_operations *ops)
> > return -EINVAL;
> >
> > INIT_LIST_HEAD(&ops->list);
> > - mutex_lock(&dyn_event_ops_mutex);
> > + guard(mutex)(&dyn_event_ops_mutex);
> > list_add_tail(&ops->list, &dyn_event_ops_list);
> > - mutex_unlock(&dyn_event_ops_mutex);
>
> I don't care for a scoped guards around simple paths. The great thing about
> guard()s is that they help prevent bugs when you have complex code between
> the lock and unlock that may need to exit.
>
> But replacing:
>
>
> mutex_lock(&dyn_event_ops_mutex);
> list_add_tail(&ops->list, &dyn_event_ops_list);
> mutex_unlock(&dyn_event_ops_mutex);
> }
>
>
> With:
>
> guard(mutex)(&dyn_event_ops_mutex);
> list_add_tail(&ops->list, &dyn_event_ops_list);
> }
>
> is overkill to me. The first one is much easier to read. The second one
> begs the question, "why did they use a guard here?"

OK. fair enough. I think I was getting a little too excited. :(

>
> > return 0;
> > }
> >
> > @@ -106,20 +105,20 @@ int dyn_event_release(const char *raw_command,
> > struct dyn_event_operations *type goto out;
> > }
> >
> > - mutex_lock(&event_mutex);
> > - for_each_dyn_event_safe(pos, n) {
> > - if (type && type != pos->ops)
> > - continue;
> > - if (!pos->ops->match(system, event,
> > - argc - 1, (const char **)argv + 1, pos))
> > - continue;
> > -
> > - ret = pos->ops->free(pos);
> > - if (ret)
> > - break;
> > + scoped_guard(mutex, &event_mutex) {
> > + for_each_dyn_event_safe(pos, n) {
> > + if (type && type != pos->ops)
> > + continue;
> > + if (!pos->ops->match(system, event,
> > + argc - 1, (const char **)argv +
> > 1, pos))
> > + continue;
> > +
> > + ret = pos->ops->free(pos);
> > + if (ret)
> > + break;
> > + }
> > + tracing_reset_all_online_cpus();
> > }
>
> This scoped_guard() doesn't give us anything. We still have the out label
> below (which my patch removes).

OK.

> > - tracing_reset_all_online_cpus();
> > - mutex_unlock(&event_mutex);
> > out:
> > argv_free(argv);
> > return ret;
> > @@ -133,13 +132,12 @@ static int create_dyn_event(const char *raw_command)
> > if (raw_command[0] == '-' || raw_command[0] == '!')
> > return dyn_event_release(raw_command, NULL);
> >
> > - mutex_lock(&dyn_event_ops_mutex);
> > + guard(mutex)(&dyn_event_ops_mutex);
> > list_for_each_entry(ops, &dyn_event_ops_list, list) {
> > ret = ops->create(raw_command);
> > if (!ret || ret != -ECANCELED)
> > break;
> > }
>
> I also don't think this helps much here.

OK.

> > - mutex_unlock(&dyn_event_ops_mutex);
> > if (ret == -ECANCELED)
> > ret = -EINVAL;
> >
> > @@ -198,7 +196,7 @@ int dyn_events_release_all(struct dyn_event_operations *type)
> > struct dyn_event *ev, *tmp;
> > int ret = 0;
> >
> > - mutex_lock(&event_mutex);
> > + guard(mutex)(&event_mutex);
> > for_each_dyn_event(ev) {
> > if (type && ev->ops != type)
> > continue;
> > @@ -216,7 +214,6 @@ int dyn_events_release_all(struct dyn_event_operations *type)
> > }
> > out:
>
> And the same issue here too. Why the guard when you still need to do the
> goto?

Yeah, we still need to call tracing_reset_all_online_cpus() here.

>
>
> > tracing_reset_all_online_cpus();
> > - mutex_unlock(&event_mutex);
> >
> > return ret;
> > }
>
>
> There's a reason I looked at this file an didn't add any guards to it (when
> I forgot that you touched it). They are all special cases, and I rather
> avoid adding special case guards to handle it. I don't believe it makes it
> any less error prone.
>
> Are you OK with dropping this patch?

Yeah, let's drop it.

Thanks for your review!

>
> -- Steve

--
Masami Hiramatsu (Google) <mhiramat@kernel.org>