[PATCH] rcu,ftrace: Fix ftrace recursion

[Peter Zijlstra <peterz@infradead.org>]

Kim reported that perf-ftrace made his box unhappy. It turns out that
commit:

  ff5c4f5cad33 ("rcu/tree: Mark the idle relevant functions noinstr")

removed one too many notrace. Probably due to there not being a helpful
comment.

Reinstate the notrace and add a comment to avoid loosing it again.

Fixes: ff5c4f5cad33 ("rcu/tree: Mark the idle relevant functions noinstr")
Reported-by: Kim Phillips <kim.phillips@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/rcu/tree.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index ee5e595501e8..33020d84ec6b 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1098,8 +1098,11 @@ noinstr bool __rcu_is_watching(void)
  * CPU can safely enter RCU read-side critical sections.  In other words,
  * if the current CPU is not in its idle loop or is in an interrupt or
  * NMI handler, return true.
+ *
+ * Must be notrace because __ftrace_ops_list_func() / ftrace_ops_assist_func()
+ * will call this (for every function) outside of recursion protection.
  */
-bool rcu_is_watching(void)
+notrace bool rcu_is_watching(void)
 {
 	bool ret;