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SubjectRe: [PATCH v9 4/7] tracepoint: Make rcuidle tracepoint callers use SRCU
On Wed, 11 Jul 2018 07:27:44 -0700
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

> On Wed, Jul 11, 2018 at 09:00:03AM -0400, Steven Rostedt wrote:
> > On Wed, 11 Jul 2018 14:49:54 +0200
> > Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > > On Thu, Jun 28, 2018 at 11:21:46AM -0700, Joel Fernandes wrote:
> > > > - it_func_ptr = rcu_dereference_sched((tp)->funcs); \
> > >
> > > I would convert to rcu_dereference_raw() to appease sparse. The fancy
> > > stuff below is pointless if you then turn off all checking.
> >
> > The problem with doing this is if we use a trace event without the
> > proper _idle() or whatever, we wont get a warning that it is used
> > incorrectly with lockdep. Or does lockdep still check if "rcu is
> > watching" with rcu_dereference_raw()?
>
> No lockdep checking is done by rcu_dereference_raw().

Correct, but I think we can do this regardless. So Joel please resend
with Peter's suggestion.

The reason being is because of this:


#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.

-- Steve
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