[PATCH v2 0/1] stop_machine: Remove stop_cpus_lock and lg_double_lock/unlock()

[Oleg Nesterov <oleg@redhat.com>]

On 11/15, Oleg Nesterov wrote:
>
> I am also going to rediff/resend my old patch which removes lglock
> from stop_machine.c, but it probably needs more discussion so I'll
> send it separately.

Please see V2. It is much simpler, and it doesn't need cond_resched().

To me this looks better than changing stop_cpus() to take all stopper
locks at once. Because this actually turns stopper->lock into another
lglock.

Yes, with this patch cpu_stop_queue_two_works() spins in busy-wait loop
if it races with stop_cpus(). But lg_double_lock() spins too, and
performance-wise I think this change is a win.

To simplify the review, let me show the code with this patch applied. The
patch simply adds "bool stop_cpus_in_progress" set by queue_stop_cpus_work()
and checked by cpu_stop_queue_two_works().

	static int cpu_stop_queue_two_works(int cpu1, struct cpu_stop_work *work1,
					    int cpu2, struct cpu_stop_work *work2)
	{
		struct cpu_stopper *stopper1 = per_cpu_ptr(&cpu_stopper, cpu1);
		struct cpu_stopper *stopper2 = per_cpu_ptr(&cpu_stopper, cpu2);
		int err;
	retry:
		spin_lock_irq(&stopper1->lock);
		spin_lock_nested(&stopper2->lock, SINGLE_DEPTH_NESTING);

		err = -ENOENT;
		if (!stopper1->enabled || !stopper2->enabled)
			goto unlock;
		/*
		 * Ensure that if we race with __stop_cpus() the stoppers won't get
		 * queued up in reverse order leading to system deadlock.
		 *
		 * We can't miss stop_cpus_in_progress if queue_stop_cpus_work() has
		 * queued a work on cpu1 but not on cpu2, we hold both locks.
		 *
		 * It can be falsely true but it is safe to spin until it is cleared,
		 * queue_stop_cpus_work() does everything under preempt_disable().
		 */
		err = -EDEADLK;
		if (unlikely(stop_cpus_in_progress))
				goto unlock;

		err = 0;
		__cpu_stop_queue_work(stopper1, work1);
		__cpu_stop_queue_work(stopper2, work2);
	unlock:
		spin_unlock(&stopper2->lock);
		spin_unlock_irq(&stopper1->lock);

		if (unlikely(err == -EDEADLK)) {
			while (stop_cpus_in_progress)
				cpu_relax();
			goto retry;
		}
		return err;
	}

	static bool queue_stop_cpus_work(const struct cpumask *cpumask,
					 cpu_stop_fn_t fn, void *arg,
					 struct cpu_stop_done *done)
	{
		struct cpu_stop_work *work;
		unsigned int cpu;
		bool queued = false;

		/*
		 * Disable preemption while queueing to avoid getting
		 * preempted by a stopper which might wait for other stoppers
		 * to enter @fn which can lead to deadlock.
		 */
		preempt_disable();
		stop_cpus_in_progress = true;
		for_each_cpu(cpu, cpumask) {
			work = &per_cpu(cpu_stopper.stop_work, cpu);
			work->fn = fn;
			work->arg = arg;
			work->done = done;
			if (cpu_stop_queue_work(cpu, work))
				queued = true;
		}
		stop_cpus_in_progress = false;
		preempt_enable();

		return queued;
	}

Oleg.