Re: [PATCH 1/3] stop_machine: ensure that a queued callback will be called before cpu_stop_park()

[Oleg Nesterov <oleg@redhat.com>]

On 10/14, Peter Zijlstra wrote:
>
> 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?

Not really.

> 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.

cpu_down() should park/kill/whatever the percpu stopper thread anyway.
And this path should clear ->enabled, it can also flush the pending
works.

And we need this anyway even if cpu_down() won't use stop_machine(),
I think.

Oleg.