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Re: [RFC][PATCH 12/13] stop_machine: Remove lglock

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On 06/22, Peter Zijlstra wrote:
>
> By having stop_two_cpus() acquire two cpu_stopper::locks we gain full
> order against the global stop_machine which takes each of these locks
> in order.

Yes, but stop_machine() locks/unlocs cpu_stopper->lock sequentially, it
never holds more than 1 ->lock, so

> +static void cpu_stop_queue_work(unsigned int cpu, struct cpu_stop_work *work)
> +{
> +	struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&stopper->lock, flags);
> +	__cpu_stop_queue_work(cpu, work);
>  	spin_unlock_irqrestore(&stopper->lock, flags);
>  }

...

>  int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void *arg)
>  {
> -	struct cpu_stop_done done;
> +	struct cpu_stopper *stopper1, *stopper2;
>  	struct cpu_stop_work work1, work2;
>  	struct multi_stop_data msdata;
> +	struct cpu_stop_done done;
> +	unsigned long flags;
> +
> +	if (cpu2 < cpu1)
> +		swap(cpu1, cpu2);

...

> +	stopper1 = per_cpu_ptr(&cpu_stopper, cpu1);
> +	stopper2 = per_cpu_ptr(&cpu_stopper, cpu2);
> +
> +	spin_lock_irqsave(&stopper1->lock, flags);
> +	spin_lock(&stopper2->lock);
> +
> +	__cpu_stop_queue_work(cpu1, &work1);
> +	__cpu_stop_queue_work(cpu2, &work2);

Suppose that stop_two_cpus(cpu1 => 0, cpu2 => 1) races with stop_machine().

	- stop_machine takes the lock on CPU 0, adds the work
	  and drops the lock

	- cpu_stop_queue_work() queues both works

	- stop_machine takes the lock on CPU 1, etc

In this case both CPU 0 and 1 will run multi_cpu_stop() but they will
use different multi_stop_data's, so they will wait for each other
forever?

Oleg.

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[RFC][PATCH 00/13] percpu rwsem -v2 Peter Zijlstra <peterz@infradead.org> - 2015-06-22 14:30 +0200
  [RFC][PATCH 01/13] rcu: Create rcu_sync infrastructure Peter Zijlstra <peterz@infradead.org> - 2015-06-22 14:30 +0200
  [RFC][PATCH 05/13] percpu-rwsem: Optimize readers and reduce global impact Peter Zijlstra <peterz@infradead.org> - 2015-06-22 14:30 +0200
    Re: [RFC][PATCH 05/13] percpu-rwsem: Optimize readers and reduce  global impact Oleg Nesterov <oleg@redhat.com> - 2015-06-23 01:10 +0200
    Re: [RFC][PATCH 05/13] percpu-rwsem: Optimize readers and reduce  global impact Tejun Heo <tj@kernel.org> - 2015-06-29 17:20 +0200
      Re: [RFC][PATCH 05/13] percpu-rwsem: Optimize readers and reduce  global impact Peter Zijlstra <peterz@infradead.org> - 2015-06-29 17:20 +0200
  [RFC][PATCH 11/13] fs/locks: Replace lg_local with a per-cpu spinlock Peter Zijlstra <peterz@infradead.org> - 2015-06-22 14:30 +0200
    Re: [RFC][PATCH 11/13] fs/locks: Replace lg_local with a per-cpu  spinlock Oleg Nesterov <oleg@redhat.com> - 2015-06-23 02:30 +0200
  [RFC][PATCH 10/13] fs/locks: Replace lg_global with a percpu-rwsem Peter Zijlstra <peterz@infradead.org> - 2015-06-22 14:30 +0200
  [RFC][PATCH 12/13] stop_machine: Remove lglock Peter Zijlstra <peterz@infradead.org> - 2015-06-22 14:30 +0200
    Re: [RFC][PATCH 12/13] stop_machine: Remove lglock Oleg Nesterov <oleg@redhat.com> - 2015-06-23 00:30 +0200
      Re: [RFC][PATCH 12/13] stop_machine: Remove lglock "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2015-06-24 04:30 +0200
        Re: [RFC][PATCH 12/13] stop_machine: Remove lglock Peter Zijlstra <peterz@infradead.org> - 2015-06-24 10:40 +0200
      Re: [RFC][PATCH 12/13] stop_machine: Remove lglock Peter Zijlstra <peterz@infradead.org> - 2015-06-24 09:40 +0200
        Re: [RFC][PATCH 12/13] stop_machine: Remove lglock Ingo Molnar <mingo@kernel.org> - 2015-06-24 10:50 +0200
  [RFC][PATCH 13/13] locking: Remove lglock Peter Zijlstra <peterz@infradead.org> - 2015-06-22 14:30 +0200
  [RFC][PATCH 03/13] rcusync: Add the CONFIG_PROVE_RCU checks Peter Zijlstra <peterz@infradead.org> - 2015-06-22 14:30 +0200
  [RFC][PATCH 07/13] sched: Reorder task_struct Peter Zijlstra <peterz@infradead.org> - 2015-06-22 14:30 +0200
  Re: [RFC][PATCH 00/13] percpu rwsem -v2 Peter Zijlstra <peterz@infradead.org> - 2015-06-22 14:40 +0200
  Re: [RFC][PATCH 00/13] percpu rwsem -v2 Linus Torvalds <torvalds@linux-foundation.org> - 2015-06-22 22:10 +0200
  Re: [RFC][PATCH 00/13] percpu rwsem -v2 Ingo Molnar <mingo@kernel.org> - 2015-06-24 10:50 +0200
    Re: [RFC][PATCH 00/13] percpu rwsem -v2 Peter Zijlstra <peterz@infradead.org> - 2015-06-24 11:10 +0200
    Re: [RFC][PATCH 00/13] percpu rwsem -v2 Daniel Wagner <daniel.wagner@bmw-carit.de> - 2015-06-24 11:20 +0200
      Re: [RFC][PATCH 00/13] percpu rwsem -v2 Linus Torvalds <torvalds@linux-foundation.org> - 2015-07-02 00:00 +0200
        Re: [RFC][PATCH 00/13] percpu rwsem -v2 Peter Zijlstra <peterz@infradead.org> - 2015-07-02 11:50 +0200

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