On Thu, Oct 08, 2015 at 04:51:31PM +0200, Oleg Nesterov wrote: > cpu_stop_queue_work() checks stopper->enabled before it queues the > work, but ->enabled == T can only guarantee cpu_stop_signal_done() > if we race with cpu_down(). > > This is not enough for stop_two_cpus() or stop_machine(), they will > deadlock if multi_cpu_stop() won't be called by one of the target > CPU's. stop_machine/stop_cpus are fine, they rely on stop_cpus_mutex. > But stop_two_cpus() has to check cpu_active() to avoid the same race > with hotplug, and this check is very unobvious and probably not even > correct if we race with cpu_up(). > > Change cpu_down() pass to clear ->enabled before cpu_stopper_thread() > flushes the pending ->works and returns with KTHREAD_SHOULD_PARK set. > > Note also that smpboot_thread_call() calls cpu_stop_unpark() which > sets enabled == T at CPU_ONLINE stage, so this CPU can't go away until > cpu_stopper_thread() is called at least once. This all means that if > cpu_stop_queue_work() succeeds, we know that work->fn() will be called.
This hard relies on the fact that cpu_down uses stop machine, right? IIRC part of the hotplug rework Thomas is doing is geared towards breaking away from stop machine. There is nothing fundamental about hot-unplug that requires stop machine. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
