On Tue, Aug 05, 2014 at 02:55:10PM -0700, Paul E. McKenney wrote:
> +/* Check for nohz_full CPUs executing in userspace. */
> +static void check_no_hz_full_tasks(void)
> +{
> +#ifdef CONFIG_NO_HZ_FULL
> +	int cpu;
> +	struct task_struct *t;
> +
> +	for_each_online_cpu(cpu) {
> +		cond_resched();
> +		rcu_read_lock();
> +		t = rcu_dynticks_task_cur(cpu);
> +		if (t == NULL || is_idle_task(t)) {
> +			rcu_read_unlock();
> +			continue;
> +		}
> +		if (ACCESS_ONCE(t->rcu_tasks_holdout))
> +			ACCESS_ONCE(t->rcu_tasks_holdout) = 0;
> +		rcu_read_unlock();
> +	}
> +#endif /* #ifdef CONFIG_NO_HZ_FULL */
> +}

That's not hotplug safe afaict, and I've no idea if someone pointed that
out already because people refuse to trim email and I can't be arsed to
wade through pages and pages of quoting.