Re: [PATCH 2/2] cpuhotplug: make get_online_cpus() scalability by using percpu counter

[Oleg Nesterov <oleg@redhat.com>]

On 04/07, Lai Jiangshan wrote:
>
> Oleg Nesterov wrote:
> > On 04/05, Oleg Nesterov wrote:
> >> On 04/05, Lai Jiangshan wrote:
> >>> 1) get_online_cpus() must be allowed to be called recursively, so I added
> >>>    get_online_cpus_nest for every task for new code.
> >> Well, iirc one of the goals of
> >>
> >> 	cpu-hotplug: replace lock_cpu_hotplug() with get_online_cpus()
> >> 	86ef5c9a8edd78e6bf92879f32329d89b2d55b5a
> >>
> >> was avoiding the new members in task_struct. I leave this up to you
> >> and Gautham.
>
> Old get_online_cpus() is read-preference, I think the goal of this ability
> is allow get_online_cpus()/put_online_cpus() to be called nested.

Sure, I understand why you added task_struct->get_online_cpus_nest.

> and use per-task counter for allowing get_online_cpus()/put_online_cpus()
> to be called nested, I think this deal is absolutely worth.

As I said, I am not going to argue. I can't justify this tradeoff.

> >>>  void put_online_cpus(void)
> >>>  {
> >>> ...
> >>> +	if (!--current->get_online_cpus_nest) {
> >>> +		preempt_disable();
> >>> +		__get_cpu_var(refcount)--;
> >>> +		if (cpu_hotplug_task)
> >>> +			wake_up_process(cpu_hotplug_task);
> >> This looks unsafe. In theory nothing protects cpu_hotplug_task from
> >> exiting if refcount_sum() becomes zero, this means wake_up_process()
> >> can hit the freed/reused/unmapped task_struct. Probably cpu_hotplug_done()
> >> needs another synhronize_sched() before return.
> >
> > Yes, I think this is true, at least in theory.
>
> preempt_disable() prevent cpu_hotplug_task from exiting.

If the cpu_down() is the caller of cpu_hotplug_begin/done, then yes.

But unless I missed something, nothing protects from cpu_up() which
takes this lock too.

Just in case... I am not saying this is really possible in practice.

Oleg.