1 files changed, 710 insertions, 65 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 1fbcc748044a..3ac6f5b0a64b 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4,6 +4,10 @@
  *  Based originally on the cpuset system, extracted by Paul Menage
  *  Copyright (C) 2006 Google, Inc
  *
+ *  Notifications support
+ *  Copyright (C) 2009 Nokia Corporation
+ *  Author: Kirill A. Shutemov
+ *
  *  Copyright notices from the original cpuset code:
  *  --------------------------------------------------
  *  Copyright (C) 2003 BULL SA.
@@ -43,6 +47,7 @@
 #include <linux/string.h>
 #include <linux/sort.h>
 #include <linux/kmod.h>
+#include <linux/module.h>
 #include <linux/delayacct.h>
 #include <linux/cgroupstats.h>
 #include <linux/hash.h>
@@ -51,15 +56,21 @@
 #include <linux/pid_namespace.h>
 #include <linux/idr.h>
 #include <linux/vmalloc.h> /* TODO: replace with more sophisticated array */
+#include <linux/eventfd.h>
+#include <linux/poll.h>
 
 #include <asm/atomic.h>
 
 static DEFINE_MUTEX(cgroup_mutex);
 
-/* Generate an array of cgroup subsystem pointers */
+/*
+ * Generate an array of cgroup subsystem pointers. At boot time, this is
+ * populated up to CGROUP_BUILTIN_SUBSYS_COUNT, and modular subsystems are
+ * registered after that. The mutable section of this array is protected by
+ * cgroup_mutex.
+ */
 #define SUBSYS(_x) &_x ## _subsys,
-
-static struct cgroup_subsys *subsys[] = {
+static struct cgroup_subsys *subsys[CGROUP_SUBSYS_COUNT] = {
 #include <linux/cgroup_subsys.h>
 };
 
@@ -146,6 +157,35 @@ struct css_id {
         unsigned short stack[0]; /* Array of Length (depth+1) */
 };
 
+/*
+ * cgroup_event represents events which userspace want to recieve.
+ */
+struct cgroup_event {
+        /*
+         * Cgroup which the event belongs to.
+         */
+        struct cgroup *cgrp;
+        /*
+         * Control file which the event associated.
+         */
+        struct cftype *cft;
+        /*
+         * eventfd to signal userspace about the event.
+         */
+        struct eventfd_ctx *eventfd;
+        /*
+         * Each of these stored in a list by the cgroup.
+         */
+        struct list_head list;
+        /*
+         * All fields below needed to unregister event when
+         * userspace closes eventfd.
+         */
+        poll_table pt;
+        wait_queue_head_t *wqh;
+        wait_queue_t wait;
+        struct work_struct remove;
+};
 
 /* The list of hierarchy roots */
 
@@ -166,6 +206,20 @@ static DEFINE_SPINLOCK(hierarchy_id_lock);
  */
 static int need_forkexit_callback __read_mostly;
 
+#ifdef CONFIG_PROVE_LOCKING
+int cgroup_lock_is_held(void)
+{
+        return lockdep_is_held(&cgroup_mutex);
+}
+#else /* #ifdef CONFIG_PROVE_LOCKING */
+int cgroup_lock_is_held(void)
+{
+        return mutex_is_locked(&cgroup_mutex);
+}
+#endif /* #else #ifdef CONFIG_PROVE_LOCKING */
+
+EXPORT_SYMBOL_GPL(cgroup_lock_is_held);
+
 /* convenient tests for these bits */
 inline int cgroup_is_removed(const struct cgroup *cgrp)
 {
@@ -235,7 +289,8 @@ struct cg_cgroup_link {
 static struct css_set init_css_set;
 static struct cg_cgroup_link init_css_set_link;
 
-static int cgroup_subsys_init_idr(struct cgroup_subsys *ss);
+static int cgroup_init_idr(struct cgroup_subsys *ss,
+                           struct cgroup_subsys_state *css);
 
 /* css_set_lock protects the list of css_set objects, and the
  * chain of tasks off each css_set.  Nests outside task->alloc_lock
@@ -433,8 +488,11 @@ static struct css_set *find_existing_css_set(
         struct hlist_node *node;
         struct css_set *cg;
 
-        /* Built the set of subsystem state objects that we want to
-         * see in the new css_set */
+        /*
+         * Build the set of subsystem state objects that we want to see in the
+         * new css_set. while subsystems can change globally, the entries here
+         * won't change, so no need for locking.
+         */
         for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                 if (root->subsys_bits & (1UL << i)) {
                         /* Subsystem is in this hierarchy. So we want
@@ -681,6 +739,7 @@ void cgroup_lock(void)
 {
         mutex_lock(&cgroup_mutex);
 }
+EXPORT_SYMBOL_GPL(cgroup_lock);
 
 /**
  * cgroup_unlock - release lock on cgroup changes
@@ -691,6 +750,7 @@ void cgroup_unlock(void)
 {
         mutex_unlock(&cgroup_mutex);
 }
+EXPORT_SYMBOL_GPL(cgroup_unlock);
 
 /*
  * A couple of forward declarations required, due to cyclic reference loop:
@@ -742,6 +802,7 @@ static int cgroup_call_pre_destroy(struct cgroup *cgrp)
                         if (ret)
                                 break;
                 }
+
         return ret;
 }
 
@@ -869,7 +930,11 @@ void cgroup_release_and_wakeup_rmdir(struct cgroup_subsys_state *css)
         css_put(css);
 }
 
-
+/*
+ * Call with cgroup_mutex held. Drops reference counts on modules, including
+ * any duplicate ones that parse_cgroupfs_options took. If this function
+ * returns an error, no reference counts are touched.
+ */
 static int rebind_subsystems(struct cgroupfs_root *root,
                               unsigned long final_bits)
 {
@@ -877,6 +942,8 @@ static int rebind_subsystems(struct cgroupfs_root *root,
         struct cgroup *cgrp = &root->top_cgroup;
         int i;
 
+        BUG_ON(!mutex_is_locked(&cgroup_mutex));
+
         removed_bits = root->actual_subsys_bits & ~final_bits;
         added_bits = final_bits & ~root->actual_subsys_bits;
         /* Check that any added subsystems are currently free */
@@ -885,6 +952,12 @@ static int rebind_subsystems(struct cgroupfs_root *root,
                 struct cgroup_subsys *ss = subsys[i];
                 if (!(bit & added_bits))
                         continue;
+                /*
+                 * Nobody should tell us to do a subsys that doesn't exist:
+                 * parse_cgroupfs_options should catch that case and refcounts
+                 * ensure that subsystems won't disappear once selected.
+                 */
+                BUG_ON(ss == NULL);
                 if (ss->root != &rootnode) {
                         /* Subsystem isn't free */
                         return -EBUSY;
@@ -904,6 +977,7 @@ static int rebind_subsystems(struct cgroupfs_root *root,
                 unsigned long bit = 1UL << i;
                 if (bit & added_bits) {
                         /* We're binding this subsystem to this hierarchy */
+                        BUG_ON(ss == NULL);
                         BUG_ON(cgrp->subsys[i]);
                         BUG_ON(!dummytop->subsys[i]);
                         BUG_ON(dummytop->subsys[i]->cgroup != dummytop);
@@ -915,8 +989,10 @@ static int rebind_subsystems(struct cgroupfs_root *root,
                         if (ss->bind)
                                 ss->bind(ss, cgrp);
                         mutex_unlock(&ss->hierarchy_mutex);
+                        /* refcount was already taken, and we're keeping it */
                 } else if (bit & removed_bits) {
                         /* We're removing this subsystem */
+                        BUG_ON(ss == NULL);
                         BUG_ON(cgrp->subsys[i] != dummytop->subsys[i]);
                         BUG_ON(cgrp->subsys[i]->cgroup != cgrp);
                         mutex_lock(&ss->hierarchy_mutex);
@@ -927,9 +1003,20 @@ static int rebind_subsystems(struct cgroupfs_root *root,
                         subsys[i]->root = &rootnode;
                         list_move(&ss->sibling, &rootnode.subsys_list);
                         mutex_unlock(&ss->hierarchy_mutex);
+                        /* subsystem is now free - drop reference on module */
+                        module_put(ss->module);
                 } else if (bit & final_bits) {
                         /* Subsystem state should already exist */
+                        BUG_ON(ss == NULL);
                         BUG_ON(!cgrp->subsys[i]);
+                        /*
+                         * a refcount was taken, but we already had one, so
+                         * drop the extra reference.
+                         */
+                        module_put(ss->module);
+#ifdef CONFIG_MODULE_UNLOAD
+                        BUG_ON(ss->module && !module_refcount(ss->module));
+#endif
                 } else {
                         /* Subsystem state shouldn't exist */
                         BUG_ON(cgrp->subsys[i]);
@@ -971,13 +1058,20 @@ struct cgroup_sb_opts {
 
 };
 
-/* Convert a hierarchy specifier into a bitmask of subsystems and
- * flags. */
-static int parse_cgroupfs_options(char *data,
-                                     struct cgroup_sb_opts *opts)
+/*
+ * Convert a hierarchy specifier into a bitmask of subsystems and flags. Call
+ * with cgroup_mutex held to protect the subsys[] array. This function takes
+ * refcounts on subsystems to be used, unless it returns error, in which case
+ * no refcounts are taken.
+ */
+static int parse_cgroupfs_options(char *data, struct cgroup_sb_opts *opts)
 {
         char *token, *o = data ?: "all";
         unsigned long mask = (unsigned long)-1;
+        int i;
+        bool module_pin_failed = false;
+
+        BUG_ON(!mutex_is_locked(&cgroup_mutex));
 
 #ifdef CONFIG_CPUSETS
         mask = ~(1UL << cpuset_subsys_id);
@@ -990,10 +1084,11 @@ static int parse_cgroupfs_options(char *data,
                         return -EINVAL;
                 if (!strcmp(token, "all")) {
                         /* Add all non-disabled subsystems */
-                        int i;
                         opts->subsys_bits = 0;
                         for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                                 struct cgroup_subsys *ss = subsys[i];
+                                if (ss == NULL)
+                                        continue;
                                 if (!ss->disabled)
                                         opts->subsys_bits |= 1ul << i;
                         }
@@ -1011,7 +1106,6 @@ static int parse_cgroupfs_options(char *data,
                         if (!opts->release_agent)
                                 return -ENOMEM;
                 } else if (!strncmp(token, "name=", 5)) {
-                        int i;
                         const char *name = token + 5;
                         /* Can't specify an empty name */
                         if (!strlen(name))
@@ -1035,9 +1129,10 @@ static int parse_cgroupfs_options(char *data,
                                 return -ENOMEM;
                 } else {
                         struct cgroup_subsys *ss;
-                        int i;
                         for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                                 ss = subsys[i];
+                                if (ss == NULL)
+                                        continue;
                                 if (!strcmp(token, ss->name)) {
                                         if (!ss->disabled)
                                                 set_bit(i, &opts->subsys_bits);
@@ -1072,9 +1167,54 @@ static int parse_cgroupfs_options(char *data,
         if (!opts->subsys_bits && !opts->name)
                 return -EINVAL;
 
+        /*
+         * Grab references on all the modules we'll need, so the subsystems
+         * don't dance around before rebind_subsystems attaches them. This may
+         * take duplicate reference counts on a subsystem that's already used,
+         * but rebind_subsystems handles this case.
+         */
+        for (i = CGROUP_BUILTIN_SUBSYS_COUNT; i < CGROUP_SUBSYS_COUNT; i++) {
+                unsigned long bit = 1UL << i;
+
+                if (!(bit & opts->subsys_bits))
+                        continue;
+                if (!try_module_get(subsys[i]->module)) {
+                        module_pin_failed = true;
+                        break;
+                }
+        }
+        if (module_pin_failed) {
+                /*
+                 * oops, one of the modules was going away. this means that we
+                 * raced with a module_delete call, and to the user this is
+                 * essentially a "subsystem doesn't exist" case.
+                 */
+                for (i--; i >= CGROUP_BUILTIN_SUBSYS_COUNT; i--) {
+                        /* drop refcounts only on the ones we took */
+                        unsigned long bit = 1UL << i;
+
+                        if (!(bit & opts->subsys_bits))
+                                continue;
+                        module_put(subsys[i]->module);
+                }
+                return -ENOENT;
+        }
+
         return 0;
 }
 
+static void drop_parsed_module_refcounts(unsigned long subsys_bits)
+{
+        int i;
+        for (i = CGROUP_BUILTIN_SUBSYS_COUNT; i < CGROUP_SUBSYS_COUNT; i++) {
+                unsigned long bit = 1UL << i;
+
+                if (!(bit & subsys_bits))
+                        continue;
+                module_put(subsys[i]->module);
+        }
+}
+
 static int cgroup_remount(struct super_block *sb, int *flags, char *data)
 {
         int ret = 0;
@@ -1091,21 +1231,19 @@ static int cgroup_remount(struct super_block *sb, int *flags, char *data)
         if (ret)
                 goto out_unlock;
 
-        /* Don't allow flags to change at remount */
-        if (opts.flags != root->flags) {
-                ret = -EINVAL;
-                goto out_unlock;
-        }
-
-        /* Don't allow name to change at remount */
-        if (opts.name && strcmp(opts.name, root->name)) {
+        /* Don't allow flags or name to change at remount */
+        if (opts.flags != root->flags ||
+            (opts.name && strcmp(opts.name, root->name))) {
                 ret = -EINVAL;
+                drop_parsed_module_refcounts(opts.subsys_bits);
                 goto out_unlock;
         }
 
         ret = rebind_subsystems(root, opts.subsys_bits);
-        if (ret)
+        if (ret) {
+                drop_parsed_module_refcounts(opts.subsys_bits);
                 goto out_unlock;
+        }
 
         /* (re)populate subsystem files */
         cgroup_populate_dir(cgrp);
@@ -1136,6 +1274,8 @@ static void init_cgroup_housekeeping(struct cgroup *cgrp)
         INIT_LIST_HEAD(&cgrp->release_list);
         INIT_LIST_HEAD(&cgrp->pidlists);
         mutex_init(&cgrp->pidlist_mutex);
+        INIT_LIST_HEAD(&cgrp->event_list);
+        spin_lock_init(&cgrp->event_list_lock);
 }
 
 static void init_cgroup_root(struct cgroupfs_root *root)
@@ -1291,7 +1431,9 @@ static int cgroup_get_sb(struct file_system_type *fs_type,
         struct cgroupfs_root *new_root;
 
         /* First find the desired set of subsystems */
+        mutex_lock(&cgroup_mutex);
         ret = parse_cgroupfs_options(data, &opts);
+        mutex_unlock(&cgroup_mutex);
         if (ret)
                 goto out_err;
 
@@ -1302,7 +1444,7 @@ static int cgroup_get_sb(struct file_system_type *fs_type,
         new_root = cgroup_root_from_opts(&opts);
         if (IS_ERR(new_root)) {
                 ret = PTR_ERR(new_root);
-                goto out_err;
+                goto drop_modules;
         }
         opts.new_root = new_root;
 
@@ -1311,7 +1453,7 @@ static int cgroup_get_sb(struct file_system_type *fs_type,
         if (IS_ERR(sb)) {
                 ret = PTR_ERR(sb);
                 cgroup_drop_root(opts.new_root);
-                goto out_err;
+                goto drop_modules;
         }
 
         root = sb->s_fs_info;
@@ -1367,6 +1509,11 @@ static int cgroup_get_sb(struct file_system_type *fs_type,
                         free_cg_links(&tmp_cg_links);
                         goto drop_new_super;
                 }
+                /*
+                 * There must be no failure case after here, since rebinding
+                 * takes care of subsystems' refcounts, which are explicitly
+                 * dropped in the failure exit path.
+                 */
 
                 /* EBUSY should be the only error here */
                 BUG_ON(ret);
@@ -1405,6 +1552,8 @@ static int cgroup_get_sb(struct file_system_type *fs_type,
                  * any) is not needed
                  */
                 cgroup_drop_root(opts.new_root);
+                /* no subsys rebinding, so refcounts don't change */
+                drop_parsed_module_refcounts(opts.subsys_bits);
         }
 
         simple_set_mnt(mnt, sb);
@@ -1414,6 +1563,8 @@ static int cgroup_get_sb(struct file_system_type *fs_type,
 
  drop_new_super:
         deactivate_locked_super(sb);
+ drop_modules:
+        drop_parsed_module_refcounts(opts.subsys_bits);
  out_err:
         kfree(opts.release_agent);
         kfree(opts.name);
@@ -1495,7 +1646,9 @@ static inline struct cftype *__d_cft(struct dentry *dentry)
 int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen)
 {
         char *start;
-        struct dentry *dentry = rcu_dereference(cgrp->dentry);
+        struct dentry *dentry = rcu_dereference_check(cgrp->dentry,
+                                                      rcu_read_lock_held() ||
+                                                      cgroup_lock_is_held());
 
         if (!dentry || cgrp == dummytop) {
                 /*
@@ -1511,13 +1664,17 @@ int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen)
         *--start = '\0';
         for (;;) {
                 int len = dentry->d_name.len;
+
                 if ((start -= len) < buf)
                         return -ENAMETOOLONG;
-                memcpy(start, cgrp->dentry->d_name.name, len);
+                memcpy(start, dentry->d_name.name, len);
                 cgrp = cgrp->parent;
                 if (!cgrp)
                         break;
-                dentry = rcu_dereference(cgrp->dentry);
+
+                dentry = rcu_dereference_check(cgrp->dentry,
+                                               rcu_read_lock_held() ||
+                                               cgroup_lock_is_held());
                 if (!cgrp->parent)
                         continue;
                 if (--start < buf)
@@ -1527,6 +1684,7 @@ int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen)
         memmove(buf, start, buf + buflen - start);
         return 0;
 }
+EXPORT_SYMBOL_GPL(cgroup_path);
 
 /**
  * cgroup_attach_task - attach task 'tsk' to cgroup 'cgrp'
@@ -1539,7 +1697,7 @@ int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen)
 int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
 {
         int retval = 0;
-        struct cgroup_subsys *ss;
+        struct cgroup_subsys *ss, *failed_ss = NULL;
         struct cgroup *oldcgrp;
         struct css_set *cg;
         struct css_set *newcg;
@@ -1553,8 +1711,16 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
         for_each_subsys(root, ss) {
                 if (ss->can_attach) {
                         retval = ss->can_attach(ss, cgrp, tsk, false);
-                        if (retval)
-                                return retval;
+                        if (retval) {
+                                /*
+                                 * Remember on which subsystem the can_attach()
+                                 * failed, so that we only call cancel_attach()
+                                 * against the subsystems whose can_attach()
+                                 * succeeded. (See below)
+                                 */
+                                failed_ss = ss;
+                                goto out;
+                        }
                 }
         }
 
@@ -1568,14 +1734,17 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
          */
         newcg = find_css_set(cg, cgrp);
         put_css_set(cg);
-        if (!newcg)
-                return -ENOMEM;
+        if (!newcg) {
+                retval = -ENOMEM;
+                goto out;
+        }
 
         task_lock(tsk);
         if (tsk->flags & PF_EXITING) {
                 task_unlock(tsk);
                 put_css_set(newcg);
-                return -ESRCH;
+                retval = -ESRCH;
+                goto out;
         }
         rcu_assign_pointer(tsk->cgroups, newcg);
         task_unlock(tsk);
@@ -1601,7 +1770,22 @@ int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
          * is no longer empty.
          */
         cgroup_wakeup_rmdir_waiter(cgrp);
-        return 0;
+out:
+        if (retval) {
+                for_each_subsys(root, ss) {
+                        if (ss == failed_ss)
+                                /*
+                                 * This subsystem was the one that failed the
+                                 * can_attach() check earlier, so we don't need
+                                 * to call cancel_attach() against it or any
+                                 * remaining subsystems.
+                                 */
+                                break;
+                        if (ss->cancel_attach)
+                                ss->cancel_attach(ss, cgrp, tsk, false);
+                }
+        }
+        return retval;
 }
 
 /*
@@ -1667,6 +1851,7 @@ bool cgroup_lock_live_group(struct cgroup *cgrp)
         }
         return true;
 }
+EXPORT_SYMBOL_GPL(cgroup_lock_live_group);
 
 static int cgroup_release_agent_write(struct cgroup *cgrp, struct cftype *cft,
                                       const char *buffer)
@@ -1935,6 +2120,16 @@ static const struct inode_operations cgroup_dir_inode_operations = {
         .rename = cgroup_rename,
 };
 
+/*
+ * Check if a file is a control file
+ */
+static inline struct cftype *__file_cft(struct file *file)
+{
+        if (file->f_dentry->d_inode->i_fop != &cgroup_file_operations)
+                return ERR_PTR(-EINVAL);
+        return __d_cft(file->f_dentry);
+}
+
 static int cgroup_create_file(struct dentry *dentry, mode_t mode,
                                 struct super_block *sb)
 {
@@ -2054,6 +2249,7 @@ int cgroup_add_file(struct cgroup *cgrp,
                 error = PTR_ERR(dentry);
         return error;
 }
+EXPORT_SYMBOL_GPL(cgroup_add_file);
 
 int cgroup_add_files(struct cgroup *cgrp,
                         struct cgroup_subsys *subsys,
@@ -2068,6 +2264,7 @@ int cgroup_add_files(struct cgroup *cgrp,
         }
         return 0;
 }
+EXPORT_SYMBOL_GPL(cgroup_add_files);
 
 /**
  * cgroup_task_count - count the number of tasks in a cgroup.
@@ -2453,7 +2650,8 @@ static struct cgroup_pidlist *cgroup_pidlist_find(struct cgroup *cgrp,
 {
         struct cgroup_pidlist *l;
         /* don't need task_nsproxy() if we're looking at ourself */
-        struct pid_namespace *ns = get_pid_ns(current->nsproxy->pid_ns);
+        struct pid_namespace *ns = current->nsproxy->pid_ns;
+
         /*
          * We can't drop the pidlist_mutex before taking the l->mutex in case
          * the last ref-holder is trying to remove l from the list at the same
@@ -2463,8 +2661,6 @@ static struct cgroup_pidlist *cgroup_pidlist_find(struct cgroup *cgrp,
         mutex_lock(&cgrp->pidlist_mutex);
         list_for_each_entry(l, &cgrp->pidlists, links) {
                 if (l->key.type == type && l->key.ns == ns) {
-                        /* found a matching list - drop the extra refcount */
-                        put_pid_ns(ns);
                         /* make sure l doesn't vanish out from under us */
                         down_write(&l->mutex);
                         mutex_unlock(&cgrp->pidlist_mutex);
@@ -2475,13 +2671,12 @@ static struct cgroup_pidlist *cgroup_pidlist_find(struct cgroup *cgrp,
         l = kmalloc(sizeof(struct cgroup_pidlist), GFP_KERNEL);
         if (!l) {
                 mutex_unlock(&cgrp->pidlist_mutex);
-                put_pid_ns(ns);
                 return l;
         }
         init_rwsem(&l->mutex);
         down_write(&l->mutex);
         l->key.type = type;
-        l->key.ns = ns;
+        l->key.ns = get_pid_ns(ns);
         l->use_count = 0; /* don't increment here */
         l->list = NULL;
         l->owner = cgrp;
@@ -2789,6 +2984,173 @@ static int cgroup_write_notify_on_release(struct cgroup *cgrp,
 }
 
 /*
+ * Unregister event and free resources.
+ *
+ * Gets called from workqueue.
+ */
+static void cgroup_event_remove(struct work_struct *work)
+{
+        struct cgroup_event *event = container_of(work, struct cgroup_event,
+                        remove);
+        struct cgroup *cgrp = event->cgrp;
+
+        event->cft->unregister_event(cgrp, event->cft, event->eventfd);
+
+        eventfd_ctx_put(event->eventfd);
+        kfree(event);
+        dput(cgrp->dentry);
+}
+
+/*
+ * Gets called on POLLHUP on eventfd when user closes it.
+ *
+ * Called with wqh->lock held and interrupts disabled.
+ */
+static int cgroup_event_wake(wait_queue_t *wait, unsigned mode,
+                int sync, void *key)
+{
+        struct cgroup_event *event = container_of(wait,
+                        struct cgroup_event, wait);
+        struct cgroup *cgrp = event->cgrp;
+        unsigned long flags = (unsigned long)key;
+
+        if (flags & POLLHUP) {
+                __remove_wait_queue(event->wqh, &event->wait);
+                spin_lock(&cgrp->event_list_lock);
+                list_del(&event->list);
+                spin_unlock(&cgrp->event_list_lock);
+                /*
+                 * We are in atomic context, but cgroup_event_remove() may
+                 * sleep, so we have to call it in workqueue.
+                 */
+                schedule_work(&event->remove);
+        }
+
+        return 0;
+}
+
+static void cgroup_event_ptable_queue_proc(struct file *file,
+                wait_queue_head_t *wqh, poll_table *pt)
+{
+        struct cgroup_event *event = container_of(pt,
+                        struct cgroup_event, pt);
+
+        event->wqh = wqh;
+        add_wait_queue(wqh, &event->wait);
+}
+
+/*
+ * Parse input and register new cgroup event handler.
+ *
+ * Input must be in format '<event_fd> <control_fd> <args>'.
+ * Interpretation of args is defined by control file implementation.
+ */
+static int cgroup_write_event_control(struct cgroup *cgrp, struct cftype *cft,
+                                      const char *buffer)
+{
+        struct cgroup_event *event = NULL;
+        unsigned int efd, cfd;
+        struct file *efile = NULL;
+        struct file *cfile = NULL;
+        char *endp;
+        int ret;
+
+        efd = simple_strtoul(buffer, &endp, 10);
+        if (*endp != ' ')
+                return -EINVAL;
+        buffer = endp + 1;
+
+        cfd = simple_strtoul(buffer, &endp, 10);
+        if ((*endp != ' ') && (*endp != '\0'))
+                return -EINVAL;
+        buffer = endp + 1;
+
+        event = kzalloc(sizeof(*event), GFP_KERNEL);
+        if (!event)
+                return -ENOMEM;
+        event->cgrp = cgrp;
+        INIT_LIST_HEAD(&event->list);
+        init_poll_funcptr(&event->pt, cgroup_event_ptable_queue_proc);
+        init_waitqueue_func_entry(&event->wait, cgroup_event_wake);
+        INIT_WORK(&event->remove, cgroup_event_remove);
+
+        efile = eventfd_fget(efd);
+        if (IS_ERR(efile)) {
+                ret = PTR_ERR(efile);
+                goto fail;
+        }
+
+        event->eventfd = eventfd_ctx_fileget(efile);
+        if (IS_ERR(event->eventfd)) {
+                ret = PTR_ERR(event->eventfd);
+                goto fail;
+        }
+
+        cfile = fget(cfd);
+        if (!cfile) {
+                ret = -EBADF;
+                goto fail;
+        }
+
+        /* the process need read permission on control file */
+        ret = file_permission(cfile, MAY_READ);
+        if (ret < 0)
+                goto fail;
+
+        event->cft = __file_cft(cfile);
+        if (IS_ERR(event->cft)) {
+                ret = PTR_ERR(event->cft);
+                goto fail;
+        }
+
+        if (!event->cft->register_event || !event->cft->unregister_event) {
+                ret = -EINVAL;
+                goto fail;
+        }
+
+        ret = event->cft->register_event(cgrp, event->cft,
+                        event->eventfd, buffer);
+        if (ret)
+                goto fail;
+
+        if (efile->f_op->poll(efile, &event->pt) & POLLHUP) {
+                event->cft->unregister_event(cgrp, event->cft, event->eventfd);
+                ret = 0;
+                goto fail;
+        }
+
+        /*
+         * Events should be removed after rmdir of cgroup directory, but before
+         * destroying subsystem state objects. Let's take reference to cgroup
+         * directory dentry to do that.
+         */
+        dget(cgrp->dentry);
+
+        spin_lock(&cgrp->event_list_lock);
+        list_add(&event->list, &cgrp->event_list);
+        spin_unlock(&cgrp->event_list_lock);
+
+        fput(cfile);
+        fput(efile);
+
+        return 0;
+
+fail:
+        if (cfile)
+                fput(cfile);
+
+        if (event && event->eventfd && !IS_ERR(event->eventfd))
+                eventfd_ctx_put(event->eventfd);
+
+        if (!IS_ERR_OR_NULL(efile))
+                fput(efile);
+
+        kfree(event);
+
+        return ret;
+}
+
+/*
  * for the common functions, 'private' gives the type of file
  */
 /* for hysterical raisins, we can't put this on the older files */
@@ -2813,6 +3175,11 @@ static struct cftype files[] = {
                 .read_u64 = cgroup_read_notify_on_release,
                 .write_u64 = cgroup_write_notify_on_release,
         },
+        {
+                .name = CGROUP_FILE_GENERIC_PREFIX "event_control",
+                .write_string = cgroup_write_event_control,
+                .mode = S_IWUGO,
+        },
 };
 
 static struct cftype cft_release_agent = {
@@ -2877,8 +3244,14 @@ static void cgroup_lock_hierarchy(struct cgroupfs_root *root)
         /* We need to take each hierarchy_mutex in a consistent order */
         int i;
 
+        /*
+         * No worry about a race with rebind_subsystems that might mess up the
+         * locking order, since both parties are under cgroup_mutex.
+         */
         for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                 struct cgroup_subsys *ss = subsys[i];
+                if (ss == NULL)
+                        continue;
                 if (ss->root == root)
                         mutex_lock(&ss->hierarchy_mutex);
         }
@@ -2890,6 +3263,8 @@ static void cgroup_unlock_hierarchy(struct cgroupfs_root *root)
 
         for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                 struct cgroup_subsys *ss = subsys[i];
+                if (ss == NULL)
+                        continue;
                 if (ss->root == root)
                         mutex_unlock(&ss->hierarchy_mutex);
         }
@@ -2936,14 +3311,17 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
 
         for_each_subsys(root, ss) {
                 struct cgroup_subsys_state *css = ss->create(ss, cgrp);
+
                 if (IS_ERR(css)) {
                         err = PTR_ERR(css);
                         goto err_destroy;
                 }
                 init_cgroup_css(css, ss, cgrp);
-                if (ss->use_id)
-                        if (alloc_css_id(ss, parent, cgrp))
+                if (ss->use_id) {
+                        err = alloc_css_id(ss, parent, cgrp);
+                        if (err)
                                 goto err_destroy;
+                }
                 /* At error, ->destroy() callback has to free assigned ID. */
         }
 
@@ -3010,11 +3388,16 @@ static int cgroup_has_css_refs(struct cgroup *cgrp)
          * synchronization other than RCU, and the subsystem linked
          * list isn't RCU-safe */
         int i;
+        /*
+         * We won't need to lock the subsys array, because the subsystems
+         * we're concerned about aren't going anywhere since our cgroup root
+         * has a reference on them.
+         */
         for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                 struct cgroup_subsys *ss = subsys[i];
                 struct cgroup_subsys_state *css;
-                /* Skip subsystems not in this hierarchy */
-                if (ss->root != cgrp->root)
+                /* Skip subsystems not present or not in this hierarchy */
+                if (ss == NULL || ss->root != cgrp->root)
                         continue;
                 css = cgrp->subsys[ss->subsys_id];
                 /* When called from check_for_release() it's possible
@@ -3088,6 +3471,7 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
         struct dentry *d;
         struct cgroup *parent;
         DEFINE_WAIT(wait);
+        struct cgroup_event *event, *tmp;
         int ret;
 
         /* the vfs holds both inode->i_mutex already */
@@ -3171,6 +3555,20 @@ again:
         set_bit(CGRP_RELEASABLE, &parent->flags);
         check_for_release(parent);
 
+        /*
+         * Unregister events and notify userspace.
+         * Notify userspace about cgroup removing only after rmdir of cgroup
+         * directory to avoid race between userspace and kernelspace
+         */
+        spin_lock(&cgrp->event_list_lock);
+        list_for_each_entry_safe(event, tmp, &cgrp->event_list, list) {
+                list_del(&event->list);
+                remove_wait_queue(event->wqh, &event->wait);
+                eventfd_signal(event->eventfd, 1);
+                schedule_work(&event->remove);
+        }
+        spin_unlock(&cgrp->event_list_lock);
+
         mutex_unlock(&cgroup_mutex);
         return 0;
 }
@@ -3205,9 +3603,198 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss)
         mutex_init(&ss->hierarchy_mutex);
         lockdep_set_class(&ss->hierarchy_mutex, &ss->subsys_key);
         ss->active = 1;
+
+        /* this function shouldn't be used with modular subsystems, since they
+         * need to register a subsys_id, among other things */
+        BUG_ON(ss->module);
 }
 
 /**
+ * cgroup_load_subsys: load and register a modular subsystem at runtime
+ * @ss: the subsystem to load
+ *
+ * This function should be called in a modular subsystem's initcall. If the
+ * subsystem is built as a module, it will be assigned a new subsys_id and set
+ * up for use. If the subsystem is built-in anyway, work is delegated to the
+ * simpler cgroup_init_subsys.
+ */
+int __init_or_module cgroup_load_subsys(struct cgroup_subsys *ss)
+{
+        int i;
+        struct cgroup_subsys_state *css;
+
+        /* check name and function validity */
+        if (ss->name == NULL || strlen(ss->name) > MAX_CGROUP_TYPE_NAMELEN ||
+            ss->create == NULL || ss->destroy == NULL)
+                return -EINVAL;
+
+        /*
+         * we don't support callbacks in modular subsystems. this check is
+         * before the ss->module check for consistency; a subsystem that could
+         * be a module should still have no callbacks even if the user isn't
+         * compiling it as one.
+         */
+        if (ss->fork || ss->exit)
+                return -EINVAL;
+
+        /*
+         * an optionally modular subsystem is built-in: we want to do nothing,
+         * since cgroup_init_subsys will have already taken care of it.
+         */
+        if (ss->module == NULL) {
+                /* a few sanity checks */
+                BUG_ON(ss->subsys_id >= CGROUP_BUILTIN_SUBSYS_COUNT);
+                BUG_ON(subsys[ss->subsys_id] != ss);
+                return 0;
+        }
+
+        /*
+         * need to register a subsys id before anything else - for example,
+         * init_cgroup_css needs it.
+         */
+        mutex_lock(&cgroup_mutex);
+        /* find the first empty slot in the array */
+        for (i = CGROUP_BUILTIN_SUBSYS_COUNT; i < CGROUP_SUBSYS_COUNT; i++) {
+                if (subsys[i] == NULL)
+                        break;
+        }
+        if (i == CGROUP_SUBSYS_COUNT) {
+                /* maximum number of subsystems already registered! */
+                mutex_unlock(&cgroup_mutex);
+                return -EBUSY;
+        }
+        /* assign ourselves the subsys_id */
+        ss->subsys_id = i;
+        subsys[i] = ss;
+
+        /*
+         * no ss->create seems to need anything important in the ss struct, so
+         * this can happen first (i.e. before the rootnode attachment).
+         */
+        css = ss->create(ss, dummytop);
+        if (IS_ERR(css)) {
+                /* failure case - need to deassign the subsys[] slot. */
+                subsys[i] = NULL;
+                mutex_unlock(&cgroup_mutex);
+                return PTR_ERR(css);
+        }
+
+        list_add(&ss->sibling, &rootnode.subsys_list);
+        ss->root = &rootnode;
+
+        /* our new subsystem will be attached to the dummy hierarchy. */
+        init_cgroup_css(css, ss, dummytop);
+        /* init_idr must be after init_cgroup_css because it sets css->id. */
+        if (ss->use_id) {
+                int ret = cgroup_init_idr(ss, css);
+                if (ret) {
+                        dummytop->subsys[ss->subsys_id] = NULL;
+                        ss->destroy(ss, dummytop);
+                        subsys[i] = NULL;
+                        mutex_unlock(&cgroup_mutex);
+                        return ret;
+                }
+        }
+
+        /*
+         * Now we need to entangle the css into the existing css_sets. unlike
+         * in cgroup_init_subsys, there are now multiple css_sets, so each one
+         * will need a new pointer to it; done by iterating the css_set_table.
+         * furthermore, modifying the existing css_sets will corrupt the hash
+         * table state, so each changed css_set will need its hash recomputed.
+         * this is all done under the css_set_lock.
+         */
+        write_lock(&css_set_lock);
+        for (i = 0; i < CSS_SET_TABLE_SIZE; i++) {
+                struct css_set *cg;
+                struct hlist_node *node, *tmp;
+                struct hlist_head *bucket = &css_set_table[i], *new_bucket;
+
+                hlist_for_each_entry_safe(cg, node, tmp, bucket, hlist) {
+                        /* skip entries that we already rehashed */
+                        if (cg->subsys[ss->subsys_id])
+                                continue;
+                        /* remove existing entry */
+                        hlist_del(&cg->hlist);
+                        /* set new value */
+                        cg->subsys[ss->subsys_id] = css;
+                        /* recompute hash and restore entry */
+                        new_bucket = css_set_hash(cg->subsys);
+                        hlist_add_head(&cg->hlist, new_bucket);
+                }
+        }
+        write_unlock(&css_set_lock);
+
+        mutex_init(&ss->hierarchy_mutex);
+        lockdep_set_class(&ss->hierarchy_mutex, &ss->subsys_key);
+        ss->active = 1;
+
+        /* success! */
+        mutex_unlock(&cgroup_mutex);
+        return 0;
+}
+EXPORT_SYMBOL_GPL(cgroup_load_subsys);
+
+/**
+ * cgroup_unload_subsys: unload a modular subsystem
+ * @ss: the subsystem to unload
+ *
+ * This function should be called in a modular subsystem's exitcall. When this
+ * function is invoked, the refcount on the subsystem's module will be 0, so
+ * the subsystem will not be attached to any hierarchy.
+ */
+void cgroup_unload_subsys(struct cgroup_subsys *ss)
+{
+        struct cg_cgroup_link *link;
+        struct hlist_head *hhead;
+
+        BUG_ON(ss->module == NULL);
+
+        /*
+         * we shouldn't be called if the subsystem is in use, and the use of
+         * try_module_get in parse_cgroupfs_options should ensure that it
+         * doesn't start being used while we're killing it off.
+         */
+        BUG_ON(ss->root != &rootnode);
+
+        mutex_lock(&cgroup_mutex);
+        /* deassign the subsys_id */
+        BUG_ON(ss->subsys_id < CGROUP_BUILTIN_SUBSYS_COUNT);
+        subsys[ss->subsys_id] = NULL;
+
+        /* remove subsystem from rootnode's list of subsystems */
+        list_del(&ss->sibling);
+
+        /*
+         * disentangle the css from all css_sets attached to the dummytop. as
+         * in loading, we need to pay our respects to the hashtable gods.
+         */
+        write_lock(&css_set_lock);
+        list_for_each_entry(link, &dummytop->css_sets, cgrp_link_list) {
+                struct css_set *cg = link->cg;
+
+                hlist_del(&cg->hlist);
+                BUG_ON(!cg->subsys[ss->subsys_id]);
+                cg->subsys[ss->subsys_id] = NULL;
+                hhead = css_set_hash(cg->subsys);
+                hlist_add_head(&cg->hlist, hhead);
+        }
+        write_unlock(&css_set_lock);
+
+        /*
+         * remove subsystem's css from the dummytop and free it - need to free
+         * before marking as null because ss->destroy needs the cgrp->subsys
+         * pointer to find their state. note that this also takes care of
+         * freeing the css_id.
+         */
+        ss->destroy(ss, dummytop);
+        dummytop->subsys[ss->subsys_id] = NULL;
+
+        mutex_unlock(&cgroup_mutex);
+}
+EXPORT_SYMBOL_GPL(cgroup_unload_subsys);
+
+/**
  * cgroup_init_early - cgroup initialization at system boot
  *
  * Initialize cgroups at system boot, and initialize any
@@ -3235,7 +3822,8 @@ int __init cgroup_init_early(void)
         for (i = 0; i < CSS_SET_TABLE_SIZE; i++)
                 INIT_HLIST_HEAD(&css_set_table[i]);
 
-        for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
+        /* at bootup time, we don't worry about modular subsystems */
+        for (i = 0; i < CGROUP_BUILTIN_SUBSYS_COUNT; i++) {
                 struct cgroup_subsys *ss = subsys[i];
 
                 BUG_ON(!ss->name);
@@ -3270,12 +3858,13 @@ int __init cgroup_init(void)
         if (err)
                 return err;
 
-        for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
+        /* at bootup time, we don't worry about modular subsystems */
+        for (i = 0; i < CGROUP_BUILTIN_SUBSYS_COUNT; i++) {
                 struct cgroup_subsys *ss = subsys[i];
                 if (!ss->early_init)
                         cgroup_init_subsys(ss);
                 if (ss->use_id)
-                        cgroup_subsys_init_idr(ss);
+                        cgroup_init_idr(ss, init_css_set.subsys[ss->subsys_id]);
         }
 
         /* Add init_css_set to the hash table */
@@ -3379,9 +3968,16 @@ static int proc_cgroupstats_show(struct seq_file *m, void *v)
         int i;
 
         seq_puts(m, "#subsys_name\thierarchy\tnum_cgroups\tenabled\n");
+        /*
+         * ideally we don't want subsystems moving around while we do this.
+         * cgroup_mutex is also necessary to guarantee an atomic snapshot of
+         * subsys/hierarchy state.
+         */
         mutex_lock(&cgroup_mutex);
         for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                 struct cgroup_subsys *ss = subsys[i];
+                if (ss == NULL)
+                        continue;
                 seq_printf(m, "%s\t%d\t%d\t%d\n",
                            ss->name, ss->root->hierarchy_id,
                            ss->root->number_of_cgroups, !ss->disabled);
@@ -3439,7 +4035,12 @@ void cgroup_fork_callbacks(struct task_struct *child)
 {
         if (need_forkexit_callback) {
                 int i;
-                for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
+                /*
+                 * forkexit callbacks are only supported for builtin
+                 * subsystems, and the builtin section of the subsys array is
+                 * immutable, so we don't need to lock the subsys array here.
+                 */
+                for (i = 0; i < CGROUP_BUILTIN_SUBSYS_COUNT; i++) {
                         struct cgroup_subsys *ss = subsys[i];
                         if (ss->fork)
                                 ss->fork(ss, child);
@@ -3508,7 +4109,11 @@ void cgroup_exit(struct task_struct *tsk, int run_callbacks)
         struct css_set *cg;
 
         if (run_callbacks && need_forkexit_callback) {
-                for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
+                /*
+                 * modular subsystems can't use callbacks, so no need to lock
+                 * the subsys array
+                 */
+                for (i = 0; i < CGROUP_BUILTIN_SUBSYS_COUNT; i++) {
                         struct cgroup_subsys *ss = subsys[i];
                         if (ss->exit)
                                 ss->exit(ss, tsk);
@@ -3702,12 +4307,13 @@ static void check_for_release(struct cgroup *cgrp)
         }
 }
 
-void __css_put(struct cgroup_subsys_state *css)
+/* Caller must verify that the css is not for root cgroup */
+void __css_put(struct cgroup_subsys_state *css, int count)
 {
         struct cgroup *cgrp = css->cgroup;
         int val;
         rcu_read_lock();
-        val = atomic_dec_return(&css->refcnt);
+        val = atomic_sub_return(count, &css->refcnt);
         if (val == 1) {
                 if (notify_on_release(cgrp)) {
                         set_bit(CGRP_RELEASABLE, &cgrp->flags);
@@ -3718,6 +4324,7 @@ void __css_put(struct cgroup_subsys_state *css)
         rcu_read_unlock();
         WARN_ON_ONCE(val < 1);
 }
+EXPORT_SYMBOL_GPL(__css_put);
 
 /*
  * Notify userspace when a cgroup is released, by running the
@@ -3799,8 +4406,11 @@ static int __init cgroup_disable(char *str)
         while ((token = strsep(&str, ",")) != NULL) {
                 if (!*token)
                         continue;
-
-                for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
+                /*
+                 * cgroup_disable, being at boot time, can't know about module
+                 * subsystems, so we don't worry about them.
+                 */
+                for (i = 0; i < CGROUP_BUILTIN_SUBSYS_COUNT; i++) {
                         struct cgroup_subsys *ss = subsys[i];
 
                         if (!strcmp(token, ss->name)) {
@@ -3824,31 +4434,65 @@ __setup("cgroup_disable=", cgroup_disable);
  */
 unsigned short css_id(struct cgroup_subsys_state *css)
 {
-        struct css_id *cssid = rcu_dereference(css->id);
+        struct css_id *cssid;
+
+        /*
+         * This css_id() can return correct value when somone has refcnt
+         * on this or this is under rcu_read_lock(). Once css->id is allocated,
+         * it's unchanged until freed.
+         */
+        cssid = rcu_dereference_check(css->id,
+                        rcu_read_lock_held() || atomic_read(&css->refcnt));
 
         if (cssid)
                 return cssid->id;
         return 0;
 }
+EXPORT_SYMBOL_GPL(css_id);
 
 unsigned short css_depth(struct cgroup_subsys_state *css)
 {
-        struct css_id *cssid = rcu_dereference(css->id);
+        struct css_id *cssid;
+
+        cssid = rcu_dereference_check(css->id,
+                        rcu_read_lock_held() || atomic_read(&css->refcnt));
 
         if (cssid)
                 return cssid->depth;
         return 0;
 }
+EXPORT_SYMBOL_GPL(css_depth);
+
+/**
+ *  css_is_ancestor - test "root" css is an ancestor of "child"
+ * @child: the css to be tested.
+ * @root: the css supporsed to be an ancestor of the child.
+ *
+ * Returns true if "root" is an ancestor of "child" in its hierarchy. Because
+ * this function reads css->id, this use rcu_dereference() and rcu_read_lock().
+ * But, considering usual usage, the csses should be valid objects after test.
+ * Assuming that the caller will do some action to the child if this returns
+ * returns true, the caller must take "child";s reference count.
+ * If "child" is valid object and this returns true, "root" is valid, too.
+ */
 
 bool css_is_ancestor(struct cgroup_subsys_state *child,
                     const struct cgroup_subsys_state *root)
 {
-        struct css_id *child_id = rcu_dereference(child->id);
-        struct css_id *root_id = rcu_dereference(root->id);
+        struct css_id *child_id;
+        struct css_id *root_id;
+        bool ret = true;
 
-        if (!child_id || !root_id || (child_id->depth < root_id->depth))
-                return false;
-        return child_id->stack[root_id->depth] == root_id->id;
+        rcu_read_lock();
+        child_id  = rcu_dereference(child->id);
+        root_id = rcu_dereference(root->id);
+        if (!child_id
+            || !root_id
+            || (child_id->depth < root_id->depth)
+            || (child_id->stack[root_id->depth] != root_id->id))
+                ret = false;
+        rcu_read_unlock();
+        return ret;
 }
 
 static void __free_css_id_cb(struct rcu_head *head)
@@ -3875,6 +4519,7 @@ void free_css_id(struct cgroup_subsys *ss, struct cgroup_subsys_state *css)
         spin_unlock(&ss->id_lock);
         call_rcu(&id->rcu_head, __free_css_id_cb);
 }
+EXPORT_SYMBOL_GPL(free_css_id);
 
 /*
  * This is called by init or create(). Then, calls to this function are
@@ -3924,15 +4569,14 @@ err_out:
 
 }
 
-static int __init cgroup_subsys_init_idr(struct cgroup_subsys *ss)
+static int __init_or_module cgroup_init_idr(struct cgroup_subsys *ss,
+                                            struct cgroup_subsys_state *rootcss)
 {
         struct css_id *newid;
-        struct cgroup_subsys_state *rootcss;
 
         spin_lock_init(&ss->id_lock);
         idr_init(&ss->idr);
 
-        rootcss = init_css_set.subsys[ss->subsys_id];
         newid = get_new_cssid(ss, 0);
         if (IS_ERR(newid))
                 return PTR_ERR(newid);
@@ -3948,13 +4592,13 @@ static int alloc_css_id(struct cgroup_subsys *ss, struct cgroup *parent,
 {
         int subsys_id, i, depth = 0;
         struct cgroup_subsys_state *parent_css, *child_css;
-        struct css_id *child_id, *parent_id = NULL;
+        struct css_id *child_id, *parent_id;
 
         subsys_id = ss->subsys_id;
         parent_css = parent->subsys[subsys_id];
         child_css = child->subsys[subsys_id];
-        depth = css_depth(parent_css) + 1;
         parent_id = parent_css->id;
+        depth = parent_id->depth + 1;
 
         child_id = get_new_cssid(ss, depth);
         if (IS_ERR(child_id))
@@ -3992,6 +4636,7 @@ struct cgroup_subsys_state *css_lookup(struct cgroup_subsys *ss, int id)
 
         return rcu_dereference(cssid->css);
 }
+EXPORT_SYMBOL_GPL(css_lookup);
 
 /**
  * css_get_next - lookup next cgroup under specified hierarchy.