Re: [RFC][mm] [PATCH 4/4] Memory cgroup hierarchy feature selector(v3)

[Li Zefan]

> +	/*
> +	 * If parent's use_hiearchy is set, we can't make any modifications
> +	 * in the child subtrees. If it is unset, then the change can
> +	 * occur, provided the current cgroup has no children.
> +	 *
> +	 * For the root cgroup, parent_mem is NULL, we allow value to be
> +	 * set if there are no children.
> +	 */
> +	if (!parent_mem || (!parent_mem->use_hierarchy &&
> +				(val == 1 || val == 0))) {
> +		if (list_empty(&cont->children))
> +			mem->use_hierarchy = val;
> +		else
> +			retval = -EBUSY;
> +	} else
> +		retval = -EINVAL;
> +
> +	return retval;
> +}

As I mentioned there is a race here. :(

echo 1 > /memcg/memory.use_hierarchy
 =>if (list_empty(&cont->children))
                                      mkdir /memcg/0
                                       => mem->use_hierarchy = 0
       mem->use_hierarchy = 1;

Now it ends up with parent's use_hierarchy is set but its child's
use_hierarchy is not set.
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