cgroup: implement cgroup_is_descendant()

A couple controllers want to determine whether two cgroups are in
ancestor/descendant relationship.  As it's more likely that the
descendant is the primary subject of interest and there are other
operations focusing on the descendants, let's ask is_descendent rather
than is_ancestor.

Implementation is trivial as the previous patch guarantees that all
ancestors of a cgroup stay accessible as long as the cgroup is
accessible.

tj: Removed depth optimization, renamed from cgroup_is_ancestor(),
    rewrote descriptions.
Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/cgroup.h

@@ -439,6 +439,7 @@ int cgroup_add_cftypes(struct cgroup_subsys *ss, struct cftype *cfts);
 int cgroup_rm_cftypes(struct cgroup_subsys *ss, struct cftype *cfts);
 
 int cgroup_is_removed(const struct cgroup *cgrp);
+bool cgroup_is_descendant(struct cgroup *cgrp, struct cgroup *ancestor);
 
 int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen);
 

kernel/cgroup.c

@@ -276,6 +276,26 @@ inline int cgroup_is_removed(const struct cgroup *cgrp)
 	return test_bit(CGRP_REMOVED, &cgrp->flags);
 }
 
+/**
+ * cgroup_is_descendant - test ancestry
+ * @cgrp: the cgroup to be tested
+ * @ancestor: possible ancestor of @cgrp
+ *
+ * Test whether @cgrp is a descendant of @ancestor.  It also returns %true
+ * if @cgrp == @ancestor.  This function is safe to call as long as @cgrp
+ * and @ancestor are accessible.
+ */
+bool cgroup_is_descendant(struct cgroup *cgrp, struct cgroup *ancestor)
+{
+	while (cgrp) {
+		if (cgrp == ancestor)
+			return true;
+		cgrp = cgrp->parent;
+	}
+	return false;
+}
+EXPORT_SYMBOL_GPL(cgroup_is_descendant);
+
 /* bits in struct cgroupfs_root flags field */
 enum {
 	ROOT_NOPREFIX,	/* mounted subsystems have no named prefix */