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

[Oleg Nesterov <oleg@redhat.com>]

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.


Lai, I didn't read this patch carefully yet (and I can't apply it to
Linus's tree). But at first glance,

>  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.

OTOH, I do not understand why the result of __get_cpu_var(refcount)
must be visible to refcount_sum() if we race with cpu_hotplug_begin(),
so it seems to me cpu_hotplug_begin() also needs synchronize_sched()
before refcount_sum().

Oleg.