time: add an offline-cpu check in add_timer_on()

Hi everyone,
I think we need a check in the function add_timer_on() to avoid adding a
timer into the timer list of an offline cpu.

This patch fixes this problem in add_timer_on().

---
 kernel/timer.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

index a05817c..6189561 100644
@@ -451,6 +451,7 @@ void add_timer_on(struct timer_list *timer, int cpu)
 	timer_stats_timer_set_start_info(timer);
 	BUG_ON(timer_pending(timer) || !timer->function);
 	spin_lock_irqsave(&base->lock, flags);
+	BUG_ON(cpu_is_offline(cpu));
 	timer_set_base(timer, base);
 	internal_add_timer(base, timer);
 	spin_unlock_irqrestore(&base->lock, flags);

1.5.3.7

No it doesn't -- it will just crash if a caller does that. If it's a real problem then you either need to fix the callers or change add_timer_on() to transparently put it onto another CPU.

I don't think it's a real problem though because CPU offlining does a stop machine and that should take care of most such races.

-Andi

No. CPU offlining does not prevent code to try inserting a timer on a offlined CPU. If there is a real problem with this we need to fix the calling code. We should not insert it somewhere else magically.

Thanks,

tglx

No, the patch is not fixing the problem. It crashes the machine.

Where did you run into this problem ?

Thanks,

tglx