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SubjectRe: [PATCH 3/9] cgroup: implement generic child / descendant walk macros
On Sat 03-11-12 01:38:29, Tejun Heo wrote:
[...]
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index cc5d2a0..8bd662c 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -2985,6 +2985,92 @@ static void cgroup_enable_task_cg_lists(void)
> write_unlock(&css_set_lock);
> }
>
> +/**
> + * cgroup_next_descendant_pre - find the next descendant for pre-order walk
> + * @pos: the current position (%NULL to initiate traversal)
> + * @cgroup: cgroup whose descendants to walk
> + *
> + * To be used by cgroup_for_each_descendant_pre(). Find the next
> + * descendant to visit for pre-order traversal of @cgroup's descendants.
> + */
> +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.

> +
> + /* visit the first child if exists */
> + next = list_first_or_null_rcu(&pos->children, struct cgroup, sibling);
> + if (next)
> + return next;
> +
> + /* no child, visit my or the closest ancestor's next sibling */
> + do {
> + next = list_entry_rcu(pos->sibling.next, struct cgroup,
> + sibling);
> + if (&next->sibling != &pos->parent->children)
> + return next;
> +
> + pos = pos->parent;
> + } while (pos != cgroup);
> +
> + return NULL;
> +}
> +EXPORT_SYMBOL_GPL(cgroup_next_descendant_pre);
[...]

--
Michal Hocko
SUSE Labs


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