On Mon, 29 Jan 2007, Oleg Nesterov wrote:
> > > 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?
>
> Because the last CPU_UP calls start_cpu_timer(), but since ->work.func != NULL
> we don't do schedule_delayed_work_on(). I think (if I am right) this is a
> slab's
> bug.
The CPU_DOWN would need to set work.func == NULL for this to work. But
then the slab does not shut down the work queues for the processor. Isnt
this another issue with workqueues? The slab would need a notification
that the workqueue for a processor was shutdown in order to set work.func
= NULL.
> I think cache_reap() is not alone, and this is not its fault.
>
> But please note another minor problem,
>
> void cache_reap(struct work_struct *unused)
> {
> ...
>
> schedule_delayed_work(&__get_cpu_var(reap_work), ...);
> }
>
> Even if smp_processor_id() was stable during the execution of cache_reap(),
> this work_struct can be moved to another CPU if CPU_DEAD happens. We can't
> avoid this, and this is correct.
Uhh.... This may not be correct in terms of how the slab operates.
> This means that __get_cpu_var(reap_work) returns a "wrong" struct
> delayed_work.
> This is absolutely harmless right now, but may be it's better to use
> container_of(unused, struct delayed_work, work).
Well seems that we have a set of unresolved issues with workqueues and cpu
hotplug.
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