diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 8ba6809..14efffe 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -49,6 +49,7 @@
 #include <linux/namei.h>
 #include <linux/smp_lock.h>
 #include <linux/pid_namespace.h>
+#include <linux/idr.h>
 
 #include <asm/atomic.h>
 
@@ -77,6 +78,9 @@
 	 */
 	unsigned long subsys_bits;
 
+	/* Unique id for this hierarchy. */
+	int hierarchy_id;
+
 	/* The bitmask of subsystems currently attached to this hierarchy */
 	unsigned long actual_subsys_bits;
 
@@ -147,6 +151,10 @@
 static LIST_HEAD(roots);
 static int root_count;
 
+static DEFINE_IDA(hierarchy_ida);
+static int next_hierarchy_id;
+static DEFINE_SPINLOCK(hierarchy_id_lock);
+
 /* dummytop is a shorthand for the dummy hierarchy's top cgroup */
 #define dummytop (&rootnode.top_cgroup)
 
@@ -264,42 +272,10 @@
  * compiled into their kernel but not actually in use */
 static int use_task_css_set_links __read_mostly;
 
-/* When we create or destroy a css_set, the operation simply
- * takes/releases a reference count on all the cgroups referenced
- * by subsystems in this css_set. This can end up multiple-counting
- * some cgroups, but that's OK - the ref-count is just a
- * busy/not-busy indicator; ensuring that we only count each cgroup
- * once would require taking a global lock to ensure that no
- * subsystems moved between hierarchies while we were doing so.
- *
- * Possible TODO: decide at boot time based on the number of
- * registered subsystems and the number of CPUs or NUMA nodes whether
- * it's better for performance to ref-count every subsystem, or to
- * take a global lock and only add one ref count to each hierarchy.
- */
-
-/*
- * unlink a css_set from the list and free it
- */
-static void unlink_css_set(struct css_set *cg)
+static void __put_css_set(struct css_set *cg, int taskexit)
 {
 	struct cg_cgroup_link *link;
 	struct cg_cgroup_link *saved_link;
-
-	hlist_del(&cg->hlist);
-	css_set_count--;
-
-	list_for_each_entry_safe(link, saved_link, &cg->cg_links,
-				 cg_link_list) {
-		list_del(&link->cg_link_list);
-		list_del(&link->cgrp_link_list);
-		kfree(link);
-	}
-}
-
-static void __put_css_set(struct css_set *cg, int taskexit)
-{
-	int i;
 	/*
 	 * Ensure that the refcount doesn't hit zero while any readers
 	 * can see it. Similar to atomic_dec_and_lock(), but for an
@@ -312,20 +288,27 @@
 		write_unlock(&css_set_lock);
 		return;
 	}
-	unlink_css_set(cg);
-	write_unlock(&css_set_lock);
 
-	rcu_read_lock();
-	for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
-		struct cgroup *cgrp = rcu_dereference(cg->subsys[i]->cgroup);
+	/* This css_set is dead. unlink it and release cgroup refcounts */
+	hlist_del(&cg->hlist);
+	css_set_count--;
+
+	list_for_each_entry_safe(link, saved_link, &cg->cg_links,
+				 cg_link_list) {
+		struct cgroup *cgrp = link->cgrp;
+		list_del(&link->cg_link_list);
+		list_del(&link->cgrp_link_list);
 		if (atomic_dec_and_test(&cgrp->count) &&
 		    notify_on_release(cgrp)) {
 			if (taskexit)
 				set_bit(CGRP_RELEASABLE, &cgrp->flags);
 			check_for_release(cgrp);
 		}
+
+		kfree(link);
 	}
-	rcu_read_unlock();
+
+	write_unlock(&css_set_lock);
 	kfree(cg);
 }
 
@@ -519,6 +502,7 @@
 				cgrp_link_list);
 	link->cg = cg;
 	link->cgrp = cgrp;
+	atomic_inc(&cgrp->count);
 	list_move(&link->cgrp_link_list, &cgrp->css_sets);
 	/*
 	 * Always add links to the tail of the list so that the list
@@ -539,7 +523,6 @@
 {
 	struct css_set *res;
 	struct cgroup_subsys_state *template[CGROUP_SUBSYS_COUNT];
-	int i;
 
 	struct list_head tmp_cg_links;
 
@@ -578,10 +561,6 @@
 
 	write_lock(&css_set_lock);
 	/* Add reference counts and links from the new css_set. */
-	for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
-		struct cgroup *cgrp = res->subsys[i]->cgroup;
-		atomic_inc(&cgrp->count);
-	}
 	list_for_each_entry(link, &oldcg->cg_links, cg_link_list) {
 		struct cgroup *c = link->cgrp;
 		if (c->root == cgrp->root)
@@ -972,8 +951,11 @@
 	unsigned long flags;
 	char *release_agent;
 	char *name;
+	/* User explicitly requested empty subsystem */
+	bool none;
 
 	struct cgroupfs_root *new_root;
+
 };
 
 /* Convert a hierarchy specifier into a bitmask of subsystems and
@@ -1002,6 +984,9 @@
 				if (!ss->disabled)
 					opts->subsys_bits |= 1ul << i;
 			}
+		} else if (!strcmp(token, "none")) {
+			/* Explicitly have no subsystems */
+			opts->none = true;
 		} else if (!strcmp(token, "noprefix")) {
 			set_bit(ROOT_NOPREFIX, &opts->flags);
 		} else if (!strncmp(token, "release_agent=", 14)) {
@@ -1051,6 +1036,8 @@
 		}
 	}
 
+	/* Consistency checks */
+
 	/*
 	 * Option noprefix was introduced just for backward compatibility
 	 * with the old cpuset, so we allow noprefix only if mounting just
@@ -1060,7 +1047,15 @@
 	    (opts->subsys_bits & mask))
 		return -EINVAL;
 
-	/* We can't have an empty hierarchy */
+
+	/* Can't specify "none" and some subsystems */
+	if (opts->subsys_bits && opts->none)
+		return -EINVAL;
+
+	/*
+	 * We either have to specify by name or by subsystems. (So all
+	 * empty hierarchies must have a name).
+	 */
 	if (!opts->subsys_bits && !opts->name)
 		return -EINVAL;
 
@@ -1141,6 +1136,31 @@
 	init_cgroup_housekeeping(cgrp);
 }
 
+static bool init_root_id(struct cgroupfs_root *root)
+{
+	int ret = 0;
+
+	do {
+		if (!ida_pre_get(&hierarchy_ida, GFP_KERNEL))
+			return false;
+		spin_lock(&hierarchy_id_lock);
+		/* Try to allocate the next unused ID */
+		ret = ida_get_new_above(&hierarchy_ida, next_hierarchy_id,
+					&root->hierarchy_id);
+		if (ret == -ENOSPC)
+			/* Try again starting from 0 */
+			ret = ida_get_new(&hierarchy_ida, &root->hierarchy_id);
+		if (!ret) {
+			next_hierarchy_id = root->hierarchy_id + 1;
+		} else if (ret != -EAGAIN) {
+			/* Can only get here if the 31-bit IDR is full ... */
+			BUG_ON(ret);
+		}
+		spin_unlock(&hierarchy_id_lock);
+	} while (ret);
+	return true;
+}
+
 static int cgroup_test_super(struct super_block *sb, void *data)
 {
 	struct cgroup_sb_opts *opts = data;
@@ -1150,8 +1170,12 @@
 	if (opts->name && strcmp(opts->name, root->name))
 		return 0;
 
-	/* If we asked for subsystems then they must match */
-	if (opts->subsys_bits && (opts->subsys_bits != root->subsys_bits))
+	/*
+	 * If we asked for subsystems (or explicitly for no
+	 * subsystems) then they must match
+	 */
+	if ((opts->subsys_bits || opts->none)
+	    && (opts->subsys_bits != root->subsys_bits))
 		return 0;
 
 	return 1;
@@ -1161,15 +1185,19 @@
 {
 	struct cgroupfs_root *root;
 
-	/* Empty hierarchies aren't supported */
-	if (!opts->subsys_bits)
+	if (!opts->subsys_bits && !opts->none)
 		return NULL;
 
 	root = kzalloc(sizeof(*root), GFP_KERNEL);
 	if (!root)
 		return ERR_PTR(-ENOMEM);
 
+	if (!init_root_id(root)) {
+		kfree(root);
+		return ERR_PTR(-ENOMEM);
+	}
 	init_cgroup_root(root);
+
 	root->subsys_bits = opts->subsys_bits;
 	root->flags = opts->flags;
 	if (opts->release_agent)
@@ -1179,6 +1207,18 @@
 	return root;
 }
 
+static void cgroup_drop_root(struct cgroupfs_root *root)
+{
+	if (!root)
+		return;
+
+	BUG_ON(!root->hierarchy_id);
+	spin_lock(&hierarchy_id_lock);
+	ida_remove(&hierarchy_ida, root->hierarchy_id);
+	spin_unlock(&hierarchy_id_lock);
+	kfree(root);
+}
+
 static int cgroup_set_super(struct super_block *sb, void *data)
 {
 	int ret;
@@ -1188,7 +1228,7 @@
 	if (!opts->new_root)
 		return -EINVAL;
 
-	BUG_ON(!opts->subsys_bits);
+	BUG_ON(!opts->subsys_bits && !opts->none);
 
 	ret = set_anon_super(sb, NULL);
 	if (ret)
@@ -1257,7 +1297,7 @@
 	sb = sget(fs_type, cgroup_test_super, cgroup_set_super, &opts);
 	if (IS_ERR(sb)) {
 		ret = PTR_ERR(sb);
-		kfree(opts.new_root);
+		cgroup_drop_root(opts.new_root);
 		goto out_err;
 	}
 
@@ -1351,7 +1391,7 @@
 		 * We re-used an existing hierarchy - the new root (if
 		 * any) is not needed
 		 */
-		kfree(opts.new_root);
+		cgroup_drop_root(opts.new_root);
 	}
 
 	simple_set_mnt(mnt, sb);
@@ -1410,7 +1450,7 @@
 	mutex_unlock(&cgroup_mutex);
 
 	kill_litter_super(sb);
-	kfree(root);
+	cgroup_drop_root(root);
 }
 
 static struct file_system_type cgroup_fs_type = {
@@ -3109,7 +3149,7 @@
 	/* Add init_css_set to the hash table */
 	hhead = css_set_hash(init_css_set.subsys);
 	hlist_add_head(&init_css_set.hlist, hhead);
-
+	BUG_ON(!init_root_id(&rootnode));
 	err = register_filesystem(&cgroup_fs_type);
 	if (err < 0)
 		goto out;
@@ -3164,7 +3204,7 @@
 		struct cgroup *cgrp;
 		int count = 0;
 
-		seq_printf(m, "%lu:", root->subsys_bits);
+		seq_printf(m, "%d:", root->hierarchy_id);
 		for_each_subsys(root, ss)
 			seq_printf(m, "%s%s", count++ ? "," : "", ss->name);
 		if (strlen(root->name))
@@ -3210,8 +3250,8 @@
 	mutex_lock(&cgroup_mutex);
 	for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
 		struct cgroup_subsys *ss = subsys[i];
-		seq_printf(m, "%s\t%lu\t%d\t%d\n",
-			   ss->name, ss->root->subsys_bits,
+		seq_printf(m, "%s\t%d\t%d\t%d\n",
+			   ss->name, ss->root->hierarchy_id,
 			   ss->root->number_of_cgroups, !ss->disabled);
 	}
 	mutex_unlock(&cgroup_mutex);
@@ -3929,8 +3969,8 @@
 			name = c->dentry->d_name.name;
 		else
 			name = "?";
-		seq_printf(seq, "Root %lu group %s\n",
-			   c->root->subsys_bits, name);
+		seq_printf(seq, "Root %d group %s\n",
+			   c->root->hierarchy_id, name);
 	}
 	rcu_read_unlock();
 	read_unlock(&css_set_lock);