On Mon, Jun 29, 2015 at 06:02:51AM +0200, Oleg Nesterov wrote: > > Btw. I can't understand the cpu_active() checks in stop_two_cpus(). > > Do we really need them? > > Ah, please ignore. > > Yes, we can't rely on stopper->enabled check in cpu_stop_queue_work(), > cpu_stop_signal_done() does not update multi_stop_data->num_threads / > ->thread_ack. So we need to ensure that cpu_online() == T for both CPUS > or multi_cpu_stop() can hang. > > But we can't use cpu_online() instead, take_cpu_down() can be already > queued. > > So this relies on the fact that CPU_DOWN_PREPARE (which removes CPU > from cpu_active_mask) is called before stop_machine(take_cpu_down) and > we do not care that cpu_active() is not stable; if we see cpu_active() > cpu_online() can't change unders us because take_cpu_down() was not > queued.
Just so. > If we change stop_two_cpus() to use stop_work_alloc_one() it can use > cpu_online(), So the one user of this actually needs cpu_active(); we do not want to go move tasks to an inactive cpu. So if you change this to cpu_online() we need to audit the user is doing the stricter test. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
