[PATCH 3/6] ftrace/x86: Warn and ignore graph tracing when RCU is disabled

All RCU disabled code should be noinstr and hence we should never get
here -- when we do, WARN about it and make sure to not actually do
tracing.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/kernel/ftrace.c |    3 +++
 1 file changed, 3 insertions(+)

--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c

@@ -646,6 +646,9 @@ void prepare_ftrace_return(unsigned long
 	if (unlikely(atomic_read(&current->tracing_graph_pause)))
 		return;
 
+	if (WARN_ONCE(!rcu_is_watching(), "RCU not on for: %pS\n", (void *)ip))
+		return;
+
 	bit = ftrace_test_recursion_trylock(ip, *parent);
 	if (bit < 0)
 		return;

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