Hello, Michal.

On Wed, Nov 07, 2012 at 05:54:57PM +0100, Michal Hocko wrote:
> > +struct cgroup *cgroup_next_descendant_pre(struct cgroup *pos,
> > +					  struct cgroup *cgroup)
> > +{
> > +	struct cgroup *next;
> > +
> > +	WARN_ON_ONCE(!rcu_read_lock_held());
> > +
> > +	/* if first iteration, pretend we just visited @cgroup */
> > +	if (!pos) {
> > +		if (list_empty(&cgroup->children))
> > +			return NULL;
> > +		pos = cgroup;
> > +	}
> 
> Is there any specific reason why the root of the tree is excluded?
> This is bit impractical because you have to special case the root
> in the code.

Yeah, thought about including it but decided against it for two
reasons.

* To be consistent with cgroup_for_each_children() - it's a bit weird
  for descendants to include self when children don't.

* Iteration root is likely to require different treatment anyway.
  e.g. for cgroup_freezer, the root is updated to the specified config
  while all the descendants inherit config from its immediate parent.
  They are different.

Thanks.

-- 
tejun