On Wed, Jul 11, 2018 at 09:22:37PM -0400, Steven Rostedt wrote:
> On Wed, 11 Jul 2018 13:56:39 -0700
> Joel Fernandes <j...@joelfernandes.org> wrote:
>
> > > > #define __DECLARE_TRACE(name, proto, args, cond, data_proto, data_args)
> > > > \
> > > > extern struct tracepoint __tracepoint_##name;
> > > > \
> > > > static inline void trace_##name(proto)
> > > > \
> > > > {
> > > > \
> > > > if (static_key_false(&__tracepoint_##name.key))
> > > > \
> > > > __DO_TRACE(&__tracepoint_##name,
> > > > \
> > > > TP_PROTO(data_proto),
> > > > \
> > > > TP_ARGS(data_args),
> > > > \
> > > > TP_CONDITION(cond), 0);
> > > > \
> > > > if (IS_ENABLED(CONFIG_LOCKDEP) && (cond)) {
> > > > \
> > > > rcu_read_lock_sched_notrace();
> > > > \
> > > >
> > > > rcu_dereference_sched(__tracepoint_##name.funcs);\
> > > > rcu_read_unlock_sched_notrace();
> > > > \
> > > > }
> > > > \
> > > > }
> > > >
> > > > Because lockdep would only trigger warnings when the tracepoint was
> > > > enabled and used in a place it shouldn't be, we added the above
> > > > IS_ENABLED(CONFIG_LOCKDEP) part to test regardless if the the
> > > > tracepoint was enabled or not. Because we do this, we don't need to
> > > > have the test in the __DO_TRACE() code itself. That means we can clean
> > > > up the code as per Peter's suggestion.
> > >
> > > Indeed, the rcu_dereference_sched() would catch it in that case, so
> > > agreed, Peter's suggestion isn't losing any debuggability.
> >
> > Hmm, but if we are doing the check later anyway, then why not do it in
> > __DO_TRACE itself?
>
> Because __DO_TRACE is only called if the trace event is enabled. If we
> never enable a trace event, we never know if it has a potential of
> doing it wrong. The second part is to trigger the warning immediately
> regardless if the trace event is enabled or not.
I see, thanks for the clarification.
> >
> > Also I guess we are discussing about changing the rcu_dereference_sched
> > which
> > I think should go into a separate patch since my patch isn't touching how
> > the
> > rcuidle==0 paths use the RCU API. So I think this is an existing issue
> > independent of this series.
>
> But the code you added made it much more complex to keep the checks as
> is. If we remove the checks then this patch doesn't need to have all
> the if statements, and we can do it the way Peter suggested.
Yes, I agree Peter's suggestion is very clean.
> But sure, go ahead and make a separate patch first that removes the
> checks from __DO_TRACE() first if you want to.
No its ok, no problem, I can just do it in the same patch now that I see the
code is much simplified with what Peter is suggesting.
thanks!
- Joel
