Re: [RFC PATCH 0/6] stop_machine: kill stop_cpus_mutex and stop_cpus_lock

[Peter Zijlstra <peterz@infradead.org>]

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.