linux-kernel - [PATCH rcu 2/6] rcu: Clarify rcu_is

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Message-Id: <20230717180317.1097590-2-paulmck@kernel.org>
Date:   Mon, 17 Jul 2023 11:03:13 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...a.com,
        rostedt@...dmis.org, "Paul E. McKenney" <paulmck@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: [PATCH rcu 2/6] rcu: Clarify rcu_is_watching() kernel-doc comment

Make it clear that this function always returns either true or false
without other planned failure modes.

Reported-by: Masami Hiramatsu <mhiramat@...nel.org>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/tree.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 1449cb69a0e0..fae9b4e29c93 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -679,10 +679,14 @@ static void rcu_disable_urgency_upon_qs(struct rcu_data *rdp)
 /**
  * rcu_is_watching - see if RCU thinks that the current CPU is not idle
  *
- * Return true if RCU is watching the running CPU, which means that this
- * 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.
+ * Return @true if RCU is watching the running CPU and @false otherwise.
+ * An @true return means that this CPU can safely enter RCU read-side
+ * critical sections.
+ *
+ * More specifically, if the current CPU is not deep within its idle
+ * loop, return @true.  Note that rcu_is_watching() will return @true if
+ * invoked from an interrupt or NMI handler, even if that interrupt or
+ * NMI interrupted the CPU while it was deep within its idle loop.
  *
  * Make notrace because it can be called by the internal functions of
  * ftrace, and making this notrace removes unnecessary recursion calls.
-- 
2.40.1