Re: slab: start_cpu_timer/cache_reap CONFIG_HOTPLUG_CPU problems

[Christoph Lameter <clameter@sgi.com>]

On Mon, 29 Jan 2007, Oleg Nesterov wrote:

> Now,
> 	static void __devinit start_cpu_timer(int cpu)
> 	{
> 		struct delayed_work *reap_work = &per_cpu(reap_work, cpu);
> 
> 		if (keventd_up() && reap_work->work.func == NULL) {
> 			init_reap_node(cpu);
> 			INIT_DELAYED_WORK(reap_work, cache_reap);
> 			schedule_delayed_work_on(cpu, reap_work,
> 						__round_jiffies_relative(HZ, cpu));
> 		}
> 	}
> 
> This is wrong. Suppose we have a CPU_UP,CPU_DOWN,CPU_UP sequence. The last
> CPU_UP will not restart a per-cpu "cache_reap timer".

Why?

> With or without recent changes, it is possible that work->func() will run on
> another CPU (not that to which it was submitted) if CPU goes down. In fact,
> this can happen while work->func() is running, so even smp_processor_id()
> is not safe to use in work->func().

But the work func was scheduled by schedule_delayed_work_on(). Isnt that a 
general problem with schedule_delayed_work_on() and keventd?

> However, cache_reap() seems to wrongly assume that smp_processor_id() is stable,
> this is the second problem.
> 
> Is my understanding correct?

cache_reap assumes that the processor id is stable based on the kevent 
thread being pinned to a processor.