cgroup: implement cgroup_rightmost_descendant()

Implement cgroup_rightmost_descendant() which returns the right most
descendant of the specified cgroup.  This can be used to skip the
cgroup's subtree while iterating with
cgroup_for_each_descendant_pre().

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Li Zefan <lizefan@huawei.com>

1 files changed, 26 insertions, 0 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 4855892798fd..6643f7053454 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -3017,6 +3017,32 @@ struct cgroup *cgroup_next_descendant_pre(struct cgroup *pos,
 }
 EXPORT_SYMBOL_GPL(cgroup_next_descendant_pre);
 
+/**
+ * cgroup_rightmost_descendant - return the rightmost descendant of a cgroup
+ * @pos: cgroup of interest
+ *
+ * Return the rightmost descendant of @pos.  If there's no descendant,
+ * @pos is returned.  This can be used during pre-order traversal to skip
+ * subtree of @pos.
+ */
+struct cgroup *cgroup_rightmost_descendant(struct cgroup *pos)
+{
+        struct cgroup *last, *tmp;
+
+        WARN_ON_ONCE(!rcu_read_lock_held());
+
+        do {
+                last = pos;
+                /* ->prev isn't RCU safe, walk ->next till the end */
+                pos = NULL;
+                list_for_each_entry_rcu(tmp, &last->children, sibling)
+                        pos = tmp;
+        } while (pos);
+
+        return last;
+}
+EXPORT_SYMBOL_GPL(cgroup_rightmost_descendant);
+
 static struct cgroup *cgroup_leftmost_descendant(struct cgroup *pos)
 {
         struct cgroup *last;