[PATCH 1/2] timers: WARN if add_timer_on is used with offlined cpu.

[Imran Khan]

timer started on an offlined cpu will not fire after
its expiry time and may never fire if that cpu remains
offline.
So add a WARN_ON_ONCE in add_timer_on, to indicate
if any of its users are (wrongly) starting a timer
on an offlined cpu.

Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Imran Khan <imran.f.khan@xxxxxxxxxx>
---
 kernel/time/timer.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index a5860bf6d16f9..ec9eb58e45241 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1370,6 +1370,12 @@ void add_timer_on(struct timer_list *timer, int cpu)
 	if (!timer->function)
 		goto out_unlock;
 
+	/*
+	 * WARN if specified cpu is offline, because on offlined cpu
+	 * timer will not fire even after its expiry.
+	 */
+	WARN_ON_ONCE(!cpu_online(cpu));
+
 	if (base != new_base) {
 		timer->flags |= TIMER_MIGRATING;
 
-- 
2.34.1