cgroup: use cgroup_setup_root() to initialize cgroup_dummy_root

cgroup_dummy_root is used to host controllers which aren't attached to
any other hierarchy.  The root is minimally set up during kernfs
bootstrap and didn't go through full hierarchy initialization.  We're
planning to use cgroup_dummy_root for the default unified hierarchy
and thus want it to be fully functional.

Replace the special initialization, which was collected into
cgroup_init() by the previous patch, with an invocation of
cgroup_setup_root().  This simplifies the init path and makes
cgroup_dummy_root a full hierarchy with its own kernfs_root and all.

As this puts the dummy hierarchy on the cgroup_roots list, rename
for_each_active_root() to for_each_root() and update its users to skip
the dummy root for now.

This patch doesn't cause any userland visible behavior changes at this
point.

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

kernel/cgroup.c

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index e66b9ee5..78017f5 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -289,8 +289,8 @@
 	for ((ssid) = 0; (ssid) < CGROUP_SUBSYS_COUNT &&		\
 	     (((ss) = cgroup_subsys[ssid]) || true); (ssid)++)
 
-/* iterate across the active hierarchies */
-#define for_each_active_root(root)					\
+/* iterate across the hierarchies */
+#define for_each_root(root)						\
 	list_for_each_entry((root), &cgroup_roots, root_list)
 
 /**
@@ -354,7 +354,6 @@
 	.mg_node		= LIST_HEAD_INIT(init_css_set.mg_node),
 };
 
-static struct cgrp_cset_link init_cgrp_cset_link;
 static int css_set_count	= 1;	/* 1 for init_css_set */
 
 /*
@@ -693,14 +692,13 @@
 	return top_cgrp->root;
 }
 
-static int cgroup_init_root_id(struct cgroupfs_root *root, int start, int end)
+static int cgroup_init_root_id(struct cgroupfs_root *root)
 {
 	int id;
 
 	lockdep_assert_held(&cgroup_mutex);
 
-	id = idr_alloc_cyclic(&cgroup_hierarchy_idr, root, start, end,
-			      GFP_KERNEL);
+	id = idr_alloc_cyclic(&cgroup_hierarchy_idr, root, 0, 0, GFP_KERNEL);
 	if (id < 0)
 		return id;
 
@@ -1405,8 +1403,7 @@
 	if (ret)
 		goto out;
 
-	/* ID 0 is reserved for dummy root, 1 for unified hierarchy */
-	ret = cgroup_init_root_id(root, 2, 0);
+	ret = cgroup_init_root_id(root);
 	if (ret)
 		goto out;
 
@@ -1486,9 +1483,12 @@
 		goto out_unlock;
 
 	/* look for a matching existing root */
-	for_each_active_root(root) {
+	for_each_root(root) {
 		bool name_match = false;
 
+		if (root == &cgroup_dummy_root)
+			continue;
+
 		/*
 		 * If we asked for a name then it must match.  Also, if
 		 * name matches but sybsys_mask doesn't, we should fail.
@@ -2106,9 +2106,12 @@
 	int retval = 0;
 
 	mutex_lock(&cgroup_mutex);
-	for_each_active_root(root) {
+	for_each_root(root) {
 		struct cgroup *from_cgrp;
 
+		if (root == &cgroup_dummy_root)
+			continue;
+
 		down_read(&css_set_rwsem);
 		from_cgrp = task_cgroup_from_root(from, root);
 		up_read(&css_set_rwsem);
@@ -4073,26 +4076,17 @@
 
 	BUG_ON(cgroup_init_cftypes(NULL, cgroup_base_files));
 
-	/* allocate id for the dummy hierarchy */
+	mutex_lock(&cgroup_tree_mutex);
 	mutex_lock(&cgroup_mutex);
 
 	/* Add init_css_set to the hash table */
 	key = css_set_hash(init_css_set.subsys);
 	hash_add(css_set_table, &init_css_set.hlist, key);
 
-	BUG_ON(cgroup_init_root_id(&cgroup_dummy_root, 0, 1));
-
-	err = idr_alloc(&cgroup_dummy_root.cgroup_idr, cgroup_dummy_top,
-			0, 1, GFP_KERNEL);
-	BUG_ON(err < 0);
-
-	cgroup_root_count = 1;
-	init_cgrp_cset_link.cset = &init_css_set;
-	init_cgrp_cset_link.cgrp = cgroup_dummy_top;
-	list_add(&init_cgrp_cset_link.cset_link, &cgroup_dummy_top->cset_links);
-	list_add(&init_cgrp_cset_link.cgrp_link, &init_css_set.cgrp_links);
+	BUG_ON(cgroup_setup_root(&cgroup_dummy_root, 0));
 
 	mutex_unlock(&cgroup_mutex);
+	mutex_unlock(&cgroup_tree_mutex);
 
 	for_each_subsys(ss, ssid) {
 		if (!ss->early_init)
@@ -4176,11 +4170,14 @@
 	mutex_lock(&cgroup_mutex);
 	down_read(&css_set_rwsem);
 
-	for_each_active_root(root) {
+	for_each_root(root) {
 		struct cgroup_subsys *ss;
 		struct cgroup *cgrp;
 		int ssid, count = 0;
 
+		if (root == &cgroup_dummy_root)
+			continue;
+
 		seq_printf(m, "%d:", root->hierarchy_id);
 		for_each_subsys(ss, ssid)
 			if (root->subsys_mask & (1 << ssid))