Messages in this thread | | | Date | Mon, 5 Apr 2010 18:29:01 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 2/2] cpuhotplug: make get_online_cpus() scalability by using percpu counter |
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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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