cgroup: separate out interface file creation from css creation

Currently, interface files are created when a css is created depending
on whether @visible is set.  This patch separates out the two into
separate steps to help code refactoring and eventually allow cgroups
which aren't visible through cgroup fs.

Move css_populate_dir() out of create_css() and drop @visible.  While
at it, rename the function to css_create() for consistency.

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

kernel/cgroup.c

@@ -222,8 +222,8 @@ static struct cftype cgroup_legacy_base_files[];
 static int rebind_subsystems(struct cgroup_root *dst_root, u16 ss_mask);
 static void css_task_iter_advance(struct css_task_iter *it);
 static int cgroup_destroy_locked(struct cgroup *cgrp);
-static int create_css(struct cgroup *cgrp, struct cgroup_subsys *ss,
-		      bool visible);
+static struct cgroup_subsys_state *css_create(struct cgroup *cgrp,
+					      struct cgroup_subsys *ss);
 static void css_release(struct percpu_ref *ref);
 static void kill_css(struct cgroup_subsys_state *css);
 static int cgroup_addrm_files(struct cgroup_subsys_state *css,
@@ -3082,14 +3082,26 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
 	 */
 	do_each_subsys_mask(ss, ssid, enable) {
 		cgroup_for_each_live_child(child, cgrp) {
-			if (css_enable & (1 << ssid))
-				ret = create_css(child, ss,
-					cgrp->subtree_control & (1 << ssid));
-			else
+			if (css_enable & (1 << ssid)) {
+				struct cgroup_subsys_state *css;
+
+				css = css_create(child, ss);
+				if (IS_ERR(css)) {
+					ret = PTR_ERR(css);
+					goto err_undo_css;
+				}
+
+				if (cgrp->subtree_control & (1 << ssid)) {
+					ret = css_populate_dir(css, NULL);
+					if (ret)
+						goto err_undo_css;
+				}
+			} else {
 				ret = css_populate_dir(cgroup_css(child, ss),
 						       NULL);
-			if (ret)
-				goto err_undo_css;
+				if (ret)
+					goto err_undo_css;
+			}
 		}
 	} while_each_subsys_mask();
 
@@ -4717,7 +4729,9 @@ static void css_release_work_fn(struct work_struct *work)
 		 * Those are supported by RCU protecting clearing of
 		 * cgrp->kn->priv backpointer.
 		 */
-		RCU_INIT_POINTER(*(void __rcu __force **)&cgrp->kn->priv, NULL);
+		if (cgrp->kn)
+			RCU_INIT_POINTER(*(void __rcu __force **)&cgrp->kn->priv,
+					 NULL);
 	}
 
 	mutex_unlock(&cgroup_mutex);
@@ -4801,17 +4815,16 @@ static void offline_css(struct cgroup_subsys_state *css)
 }
 
 /**
- * create_css - create a cgroup_subsys_state
+ * css_create - create a cgroup_subsys_state
  * @cgrp: the cgroup new css will be associated with
  * @ss: the subsys of new css
- * @visible: whether to create control knobs for the new css or not
  *
  * Create a new css associated with @cgrp - @ss pair.  On success, the new
- * css is online and installed in @cgrp with all interface files created if
- * @visible.  Returns 0 on success, -errno on failure.
+ * css is online and installed in @cgrp.  This function doesn't create the
+ * interface files.  Returns 0 on success, -errno on failure.
  */
-static int create_css(struct cgroup *cgrp, struct cgroup_subsys *ss,
-		      bool visible)
+static struct cgroup_subsys_state *css_create(struct cgroup *cgrp,
+					      struct cgroup_subsys *ss)
 {
 	struct cgroup *parent = cgroup_parent(cgrp);
 	struct cgroup_subsys_state *parent_css = cgroup_css(parent, ss);
@@ -4822,7 +4835,7 @@ static int create_css(struct cgroup *cgrp, struct cgroup_subsys *ss,
 
 	css = ss->css_alloc(parent_css);
 	if (IS_ERR(css))
-		return PTR_ERR(css);
+		return css;
 
 	init_and_link_css(css, ss, cgrp);
 
@@ -4835,12 +4848,6 @@ static int create_css(struct cgroup *cgrp, struct cgroup_subsys *ss,
 		goto err_free_percpu_ref;
 	css->id = err;
 
-	if (visible) {
-		err = css_populate_dir(css, NULL);
-		if (err)
-			goto err_free_id;
-	}
-
 	/* @css is ready to be brought online now, make it visible */
 	list_add_tail_rcu(&css->sibling, &parent_css->children);
 	cgroup_idr_replace(&ss->css_idr, css, css->id);
@@ -4858,18 +4865,16 @@ static int create_css(struct cgroup *cgrp, struct cgroup_subsys *ss,
 		ss->warned_broken_hierarchy = true;
 	}
 
-	return 0;
+	return css;
 
 err_list_del:
 	list_del_rcu(&css->sibling);
-	css_clear_dir(css, NULL);
-err_free_id:
 	cgroup_idr_remove(&ss->css_idr, css->id);
 err_free_percpu_ref:
 	percpu_ref_exit(&css->refcnt);
 err_free_css:
 	call_rcu(&css->rcu_head, css_free_rcu_fn);
-	return err;
+	return ERR_PTR(err);
 }
 
 static int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name,
@@ -4966,10 +4971,19 @@ static int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name,
 
 	/* let's create and online css's */
 	do_each_subsys_mask(ss, ssid, parent->subtree_ss_mask) {
-		ret = create_css(cgrp, ss,
-				 parent->subtree_control & (1 << ssid));
-		if (ret)
+		struct cgroup_subsys_state *css;
+
+		css = css_create(cgrp, ss);
+		if (IS_ERR(css)) {
+			ret = PTR_ERR(css);
 			goto out_destroy;
+		}
+
+		if (parent->subtree_control & (1 << ssid)) {
+			ret = css_populate_dir(css, NULL);
+			if (ret)
+				goto out_destroy;
+		}
 	} while_each_subsys_mask();
 
 	/*