2 files changed, 39 insertions, 0 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 3a04aeb8b5a1..9fd99f5e699f 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -910,6 +910,8 @@ void css_task_iter_end(struct css_task_iter *it);
 int cgroup_attach_task_all(struct task_struct *from, struct task_struct *);
 int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from);
 
+struct cgroup_subsys_state *cgroup_get_e_css(struct cgroup *cgroup,
+                                             struct cgroup_subsys *ss);
 struct cgroup_subsys_state *css_tryget_online_from_dir(struct dentry *dentry,
                                                        struct cgroup_subsys *ss);
 
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 69f033582a1a..bb263d0caab3 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -277,6 +277,10 @@ static struct cgroup_subsys_state *cgroup_e_css(struct cgroup *cgrp,
         if (!(cgrp->root->subsys_mask & (1 << ss->id)))
                 return NULL;
 
+        /*
+         * This function is used while updating css associations and thus
+         * can't test the csses directly.  Use ->child_subsys_mask.
+         */
         while (cgroup_parent(cgrp) &&
                !(cgroup_parent(cgrp)->child_subsys_mask & (1 << ss->id)))
                 cgrp = cgroup_parent(cgrp);
@@ -284,6 +288,39 @@ static struct cgroup_subsys_state *cgroup_e_css(struct cgroup *cgrp,
         return cgroup_css(cgrp, ss);
 }
 
+/**
+ * cgroup_get_e_css - get a cgroup's effective css for the specified subsystem
+ * @cgrp: the cgroup of interest
+ * @ss: the subsystem of interest
+ *
+ * Find and get the effective css of @cgrp for @ss.  The effective css is
+ * defined as the matching css of the nearest ancestor including self which
+ * has @ss enabled.  If @ss is not mounted on the hierarchy @cgrp is on,
+ * the root css is returned, so this function always returns a valid css.
+ * The returned css must be put using css_put().
+ */
+struct cgroup_subsys_state *cgroup_get_e_css(struct cgroup *cgrp,
+                                             struct cgroup_subsys *ss)
+{
+        struct cgroup_subsys_state *css;
+
+        rcu_read_lock();
+
+        do {
+                css = cgroup_css(cgrp, ss);
+
+                if (css && css_tryget_online(css))
+                        goto out_unlock;
+                cgrp = cgroup_parent(cgrp);
+        } while (cgrp);
+
+        css = init_css_set.subsys[ss->id];
+        css_get(css);
+out_unlock:
+        rcu_read_unlock();
+        return css;
+}
+
 /* convenient tests for these bits */
 static inline bool cgroup_is_dead(const struct cgroup *cgrp)
 {