cgroup: separate out cset_group_from_root() from task_cgroup_from_root()

This will be used by the planned migration path update.

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

kernel/cgroup.c

@@ -758,25 +758,15 @@ static void cgroup_destroy_root(struct cgroupfs_root *root)
 	cgroup_free_root(root);
 }
 
-/*
- * Return the cgroup for "task" from the given hierarchy. Must be
- * called with cgroup_mutex and css_set_rwsem held.
- */
-static struct cgroup *task_cgroup_from_root(struct task_struct *task,
+/* look up cgroup associated with given css_set on the specified hierarchy */
+static struct cgroup *cset_cgroup_from_root(struct css_set *cset,
 					    struct cgroupfs_root *root)
 {
-	struct css_set *cset;
 	struct cgroup *res = NULL;
 
 	lockdep_assert_held(&cgroup_mutex);
 	lockdep_assert_held(&css_set_rwsem);
 
-	/*
-	 * No need to lock the task - since we hold cgroup_mutex the
-	 * task can't change groups, so the only thing that can happen
-	 * is that it exits and its css is set back to init_css_set.
-	 */
-	cset = task_css_set(task);
 	if (cset == &init_css_set) {
 		res = &root->top_cgroup;
 	} else {
@@ -796,6 +786,21 @@ static struct cgroup *task_cgroup_from_root(struct task_struct *task,
 	return res;
 }
 
+/*
+ * Return the cgroup for "task" from the given hierarchy. Must be
+ * called with cgroup_mutex and css_set_rwsem held.
+ */
+static struct cgroup *task_cgroup_from_root(struct task_struct *task,
+					    struct cgroupfs_root *root)
+{
+	/*
+	 * No need to lock the task - since we hold cgroup_mutex the
+	 * task can't change groups, so the only thing that can happen
+	 * is that it exits and its css is set back to init_css_set.
+	 */
+	return cset_cgroup_from_root(task_css_set(task), root);
+}
+
 /*
  * There is one global cgroup mutex. We also require taking
  * task_lock() when dereferencing a task's cgroup subsys pointers.