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

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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.

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[RFC][PATCH] sched: Start stopper early Peter Zijlstra <peterz@infradead.org> - 2015-10-07 10:50 +0200
  Re: [RFC][PATCH] sched: Start stopper early Oleg Nesterov <oleg@redhat.com> - 2015-10-07 14:40 +0200
    Re: [RFC][PATCH] sched: Start stopper early Peter Zijlstra <peterz@infradead.org> - 2015-10-07 14:40 +0200
      Re: [RFC][PATCH] sched: Start stopper early Oleg Nesterov <oleg@redhat.com> - 2015-10-07 15:30 +0200
        Re: [RFC][PATCH] sched: Start stopper early Oleg Nesterov <oleg@redhat.com> - 2015-10-07 15:30 +0200
        Re: Re: [RFC][PATCH] sched: Start stopper early kbuild test robot <lkp@intel.com> - 2015-10-07 15:40 +0200
  [PATCH 0/3] (Was: [RFC][PATCH] sched: Start stopper early) Oleg Nesterov <oleg@redhat.com> - 2015-10-08 17:00 +0200
    [PATCH 1/3] stop_machine: ensure that a queued callback will be  called before cpu_stop_park() Oleg Nesterov <oleg@redhat.com> - 2015-10-08 17:00 +0200
      Re: [PATCH 1/3] stop_machine: ensure that a queued callback will be  called before cpu_stop_park() Peter Zijlstra <peterz@infradead.org> - 2015-10-14 17:40 +0200
        Re: [PATCH 1/3] stop_machine: ensure that a queued callback will  be called before cpu_stop_park() Oleg Nesterov <oleg@redhat.com> - 2015-10-14 21:10 +0200
          Re: [PATCH 1/3] stop_machine: ensure that a queued callback will be  called before cpu_stop_park() Peter Zijlstra <peterz@infradead.org> - 2015-10-14 22:40 +0200
            Re: [PATCH 1/3] stop_machine: ensure that a queued callback will  be called before cpu_stop_park() Oleg Nesterov <oleg@redhat.com> - 2015-10-15 19:10 +0200
              Re: [PATCH 1/3] stop_machine: ensure that a queued callback will be  called before cpu_stop_park() Peter Zijlstra <peterz@infradead.org> - 2015-10-16 13:00 +0200
    [PATCH 3/3] stop_machine: change cpu_stop_queue_two_works() to  rely on stopper->enabled Oleg Nesterov <oleg@redhat.com> - 2015-10-08 17:00 +0200
      Re: [PATCH 3/3] stop_machine: change cpu_stop_queue_two_works() to  rely on stopper->enabled Peter Zijlstra <peterz@infradead.org> - 2015-10-08 17:10 +0200
        Re: [PATCH 3/3] stop_machine: change cpu_stop_queue_two_works() to  rely on stopper->enabled Oleg Nesterov <oleg@redhat.com> - 2015-10-08 18:10 +0200
          Re: [PATCH 3/3] stop_machine: change cpu_stop_queue_two_works() to  rely on stopper->enabled Oleg Nesterov <oleg@redhat.com> - 2015-10-08 18:20 +0200
      [PATCH v2 3/3] stop_machine: change cpu_stop_queue_two_works() to  rely on stopper->enabled Oleg Nesterov <oleg@redhat.com> - 2015-10-08 19:10 +0200
        Re: [PATCH v2 3/3] stop_machine: change cpu_stop_queue_two_works()  to rely on stopper->enabled Peter Zijlstra <peterz@infradead.org> - 2015-10-09 18:40 +0200
          Re: [PATCH v2 3/3] stop_machine: change cpu_stop_queue_two_works()  to rely on stopper->enabled Oleg Nesterov <oleg@redhat.com> - 2015-10-09 18:50 +0200
    [PATCH 2/3] stop_machine: introduce __cpu_stop_queue_work() and  cpu_stop_queue_two_works() Oleg Nesterov <oleg@redhat.com> - 2015-10-08 17:00 +0200
  Re: [RFC][PATCH] sched: Start stopper early Oleg Nesterov <oleg@redhat.com> - 2015-10-08 20:10 +0200
    Re: [RFC][PATCH] sched: Start stopper early Oleg Nesterov <oleg@redhat.com> - 2015-10-08 21:00 +0200
  [PATCH 0/3] make stopper threads more "selfparking" Oleg Nesterov <oleg@redhat.com> - 2015-10-09 18:10 +0200
    [PATCH 2/3] stop_machine: kill cpu_stop_threads->setup() and  cpu_stop_unpark() Oleg Nesterov <oleg@redhat.com> - 2015-10-09 18:10 +0200
    [PATCH 3/3] sched: start stopper early Oleg Nesterov <oleg@redhat.com> - 2015-10-09 18:10 +0200
      Re: [PATCH 3/3] sched: start stopper early Oleg Nesterov <oleg@redhat.com> - 2015-10-09 19:00 +0200
    [PATCH 1/3] stop_machine: kill smp_hotplug_thread->pre_unpark,  introduce stop_machine_unpark() Oleg Nesterov <oleg@redhat.com> - 2015-10-09 18:10 +0200
  Re: [RFC][PATCH] sched: Start stopper early Heiko Carstens <heiko.carstens@de.ibm.com> - 2015-10-16 10:30 +0200
    Re: [RFC][PATCH] sched: Start stopper early Peter Zijlstra <peterz@infradead.org> - 2015-10-16 12:00 +0200
      Re: [RFC][PATCH] sched: Start stopper early Heiko Carstens <heiko.carstens@de.ibm.com> - 2015-10-16 14:10 +0200

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