CONFIG_GENERIC_ENTRY disallows any and all tracing when RCU isn't
enabled.

XXX if s390 (the only other GENERIC_ENTRY user as of this writing)
isn't comfortable with this, we could switch to
HAVE_NOINSTR_VALIDATION which is x86_64 only atm.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 include/linux/tracepoint.h |   13 ++++++++++++-
 kernel/trace/trace.c       |    3 +++
 2 files changed, 15 insertions(+), 1 deletion(-)

--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -178,6 +178,16 @@ static inline struct tracepoint *tracepo
 #endif /* CONFIG_HAVE_STATIC_CALL */
 
 /*
+ * CONFIG_GENERIC_ENTRY archs are expected to have sanitized entry and idle
+ * code that disallow any/all tracing/instrumentation when RCU isn't watching.
+ */
+#ifdef CONFIG_GENERIC_ENTRY
+#define RCUIDLE_COND(rcuidle)	(rcuidle)
+#else
+#define RCUIDLE_COND(rcuidle)	(rcuidle && in_nmi())
+#endif
+
+/*
  * it_func[0] is never NULL because there is at least one element in the array
  * when the array itself is non NULL.
  */
@@ -189,7 +199,8 @@ static inline struct tracepoint *tracepo
 			return;						\
 									\
 		/* srcu can't be used from NMI */			\
-		WARN_ON_ONCE(rcuidle && in_nmi());			\
+		if (WARN_ON_ONCE(RCUIDLE_COND(rcuidle)))		\
+			return;						\
 									\
 		/* keep srcu and sched-rcu usage consistent */		\
 		preempt_disable_notrace();				\
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -3104,6 +3104,9 @@ void __trace_stack(struct trace_array *t
 		return;
 	}
 
+	if (WARN_ON_ONCE(IS_ENABLED(CONFIG_GENERIC_ENTRY)))
+		return;
+
 	/*
 	 * When an NMI triggers, RCU is enabled via rcu_nmi_enter(),
 	 * but if the above rcu_is_watching() failed, then the NMI