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

From: Oleg Nesterov
Date: Mon Apr 05 2010 - 13:04:46 EST

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()

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().


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