Messages in this thread | | | Date | Mon, 29 Jun 2015 10:51:27 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC PATCH 0/6] stop_machine: kill stop_cpus_mutex and stop_cpus_lock |
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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.
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