[PATCH] nohz: Reset next_tick cache even when the timer has no regs

[Frederic Weisbecker]

The tick IRQ regs can be NULL if hrtimer_interrupt() is called from
non-interrupt contexts (ex: hotplug CPU down). For such very special
path we forget to clean the cached next tick deadline. If we are in
dynticks mode and the actual timer deadline is ahead of us, we might
perform a buggy bypass of the next clock reprogramming.

In fact since CPU down is the only user I'm aware of, this fix is likely
unnecessary as dying CPUs already clean their tick deadline cache. But
given how hard it is to debug such timer cache related issue, we should
never be short on paranoid measures.

Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 kernel/time/tick-sched.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 764d290..ed18ca5 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -1200,8 +1200,17 @@ static enum hrtimer_restart tick_sched_timer(struct hrtimer *timer)
 	 * Do not call, when we are not in irq context and have
 	 * no valid regs pointer
 	 */
-	if (regs)
+	if (regs) {
 		tick_sched_handle(ts, regs);
+	} else {
+		/*
+		 * IRQ regs are NULL if hrtimer_interrupt() is called from
+		 * non-interrupt contexts (ex: hotplug cpu down). Make sure to
+		 * clean the cached next tick deadline to avoid buggy bypass of
+		 * clock reprog.
+		 */
+		ts->next_tick = 0;
+	}
 
 	/* No need to reprogram if we are in idle or full dynticks mode */
 	if (unlikely(ts->tick_stopped))
-- 
2.7.4