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

[David Carlier <devnexen@gmail.com>]

trace_call__##name() was added by commit 677a3d82b640 ("tracepoint: Add
trace_call__##name() API") for callers that already gate on
trace_##name##_enabled(), letting them skip the redundant
static_branch_unlikely() re-evaluation. The LOCKDEP rcu_is_watching()
WARN_ONCE() that trace_##name() carries on the disabled path was not
mirrored, so the cold-path coverage added by commit c2679254b9c9
("tracing: Make tracepoint lockdep check actually test something") is
lost for trace_call__##name() callers when the tracepoint is disabled.

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. The natural place is the gate that
trace_call__##name() callers consult: trace_##name##_enabled().

Fixes: 677a3d82b640 ("tracepoint: Add trace_call__##name() API")
Cc: Vineeth Pillai (Google) <vineeth@bitbyteword.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: David Carlier <devnexen@gmail.com>
---
 include/linux/tracepoint.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index 578e520b6ee6..3b3ab19c2627 100644
--- 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);\
 	}
 
-- 
2.53.0