Some users of hrtimer need to change the callback function after the
initial setup, and they touch the 'function' field of struct hrtimer
directly. However, directly changing the 'function' field is not safe under
all circumstances. It's only safe when the hrtimer is not enqueued.

Introduce hrtimer_update_function() helper function, which also performs
runtime checks whether it is safe (i.e. the timer is not enqueued).

The field 'function' of struct hrtimer will be changed to private in the
future.

Signed-off-by: Nam Cao <namcao@linutronix.de>
---
 include/linux/hrtimer.h | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/include/linux/hrtimer.h b/include/linux/hrtimer.h
index 48872a2b4071..6e026730e803 100644
--- a/include/linux/hrtimer.h
+++ b/include/linux/hrtimer.h
@@ -327,6 +327,28 @@ static inline int hrtimer_callback_running(struct hrtimer *timer)
 	return timer->base->running == timer;
 }
 
+/**
+ * hrtimer_update_function - Update the timer's callback function
+ * @timer:	Timer to update
+ * @function:	New callback function
+ *
+ * Only safe to call if the timer is not enqueued. Can be called in the callback function if the
+ * timer is not enqueued at the same time (see the comments above HRTIMER_STATE_ENQUEUED).
+ */
+static inline void hrtimer_update_function(struct hrtimer *timer,
+					   enum hrtimer_restart (*function)(struct hrtimer *))
+{
+	guard(raw_spinlock_irqsave)(&timer->base->cpu_base->lock);
+
+	if (WARN_ON_ONCE(hrtimer_is_queued(timer)))
+		return;
+
+	if (WARN_ON_ONCE(!function))
+		return;
+
+	timer->function = function;
+}
+
 /* Forward a hrtimer so it expires after now: */
 extern u64
 hrtimer_forward(struct hrtimer *timer, ktime_t now, ktime_t interval);
-- 
2.39.5