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()
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.
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