Re: [PATCH v2] tracepoint: add lockdep rcu_is_watching() check to trace_##name##_enabled()

[Steven Rostedt <rostedt@goodmis.org>]

[ Replying to Sashiko ]

> commit 7596bdcd9abf9a26b3964f10b5d5de3249e731e9
> Author: David Carlier <devnexen@gmail.com>
> tracepoint: add lockdep rcu_is_watching() check to trace_##name##_enabled()
> This commit attempts to restore RCU lockdep coverage for trace_call__##name()
> callers by adding a warning directly to the trace_##name##_enabled() static
> branch check.
> > When the tracepoint is enabled, the rcu_dereference inside
> > __DO_TRACE_CALL() already trips under PROVE_RCU, so the warning is
> > only needed on the !enabled path.
> Is this description accurate regarding __DO_TRACE_CALL()?
> Looking at the implementation, it uses rcu_dereference_raw() which
> intentionally bypasses lockdep checking. This means trace_call__##name()
> lacks lockdep coverage on both the enabled and disabled paths.
> > diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> > --- a/include/linux/tracepoint.h
> > +++ b/include/linux/tracepoint.h
> > @@ -293,6 +293,10 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
> >  	static inline bool						\
> >  	trace_##name##_enabled(void)					\
> >  	{								\
> > +		if (IS_ENABLED(CONFIG_LOCKDEP)) {			\
> > +			WARN_ONCE(!rcu_is_watching(),			\
> > +				  "RCU not watching for tracepoint");	\
> > +		}							\
> >  		return static_branch_unlikely(&__tracepoint_##name.key);\
> >  	}

> Does this unconditional check cause spurious lockdep splats?
> Unlike the original trace_##name() macro, trace_##name##_enabled() takes no
> arguments and cannot evaluate the tracepoint's condition, such as
> cpu_online(raw_smp_processor_id()). If a subsystem queries
> trace_##name##_enabled() on an offline CPU where RCU is not watching, won't
> this trigger a false positive warning, even if the tracepoint itself would
> have been safely skipped by its condition?

If the CPU is offline, you shouldn't be using trace_##name##_enabled() to
begin with.


> Furthermore, is it already safe to query a static branch outside of an RCU
> watch window? Subsystems sometimes use this boolean query in RCU-idle
> contexts simply to skip data gathering without ever intending to execute
> the tracepoint.

The entire point of this patch is to make sure lockdep *always* checks that
"RCU is watching" at every trace_##name##_enabled() location regardless if
the tracepoint is active or not. It's not about the static branch, but the
tracepoint that is hidden behind it.


> Would it be more reliable to add the WARN_ONCE check, guarded by its cond
> parameter, directly to trace_call__##name() to appropriately mirror how
> trace_##name() behaves?

Hmm, if this does become an issue, that may be worth doing. But currently,
I do not believe the enabled() code is used by anything with conditional
tracepoints.

-- Steve