Task Control Groups: automatic userspace notification of idle cgroups

Add the following files to the cgroup filesystem:

notify_on_release - configures/reports whether the cgroup subsystem should
attempt to run a release script when this cgroup becomes unused

release_agent - configures/reports the release agent to be used for this
hierarchy (top level in each hierarchy only)

releasable - reports whether this cgroup would have been auto-released if
notify_on_release was true and a release agent was configured (mainly useful
for debugging)

To avoid locking issues, invoking the userspace release agent is done via a
workqueue task; cgroups that need to have their release agents invoked by
the workqueue task are linked on to a list.

[pj@sgi.com: Need to include kmod.h]
Signed-off-by: Paul Menage <menage@google.com>
Cc: Serge E. Hallyn <serue@us.ibm.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Dave Hansen <haveblue@us.ibm.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Paul Jackson <pj@sgi.com>
Cc: Kirill Korotaev <dev@openvz.org>
Cc: Herbert Poetzl <herbert@13thfloor.at>
Cc: Srivatsa Vaddagiri <vatsa@in.ibm.com>
Cc: Cedric Le Goater <clg@fr.ibm.com>
Signed-off-by: Paul Jackson <pj@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/cgroup.h

@@ -77,10 +77,11 @@ static inline void css_get(struct cgroup_subsys_state *css)
  * css_get()
  */
 
+extern void __css_put(struct cgroup_subsys_state *css);
 static inline void css_put(struct cgroup_subsys_state *css)
 {
 	if (!test_bit(CSS_ROOT, &css->flags))
-		atomic_dec(&css->refcnt);
+		__css_put(css);
 }
 
 struct cgroup {
@@ -112,6 +113,13 @@ struct cgroup {
 	 * tasks in this cgroup. Protected by css_set_lock
 	 */
 	struct list_head css_sets;
+
+	/*
+	 * Linked list running through all cgroups that can
+	 * potentially be reaped by the release agent. Protected by
+	 * release_list_lock
+	 */
+	struct list_head release_list;
 };
 
 /* A css_set is a structure holding pointers to a set of
@@ -293,7 +301,6 @@ struct task_struct *cgroup_iter_next(struct cgroup *cont,
 					struct cgroup_iter *it);
 void cgroup_iter_end(struct cgroup *cont, struct cgroup_iter *it);
 
-
 #else /* !CONFIG_CGROUPS */
 
 static inline int cgroup_init_early(void) { return 0; }

kernel/cgroup.c

@@ -43,8 +43,11 @@
 #include <linux/spinlock.h>
 #include <linux/string.h>
 #include <linux/sort.h>
+#include <linux/kmod.h>
 #include <asm/atomic.h>
 
+static DEFINE_MUTEX(cgroup_mutex);
+
 /* Generate an array of cgroup subsystem pointers */
 #define SUBSYS(_x) &_x ## _subsys,
 
@@ -83,6 +86,13 @@ struct cgroupfs_root {
 
 	/* Hierarchy-specific flags */
 	unsigned long flags;
+
+	/* The path to use for release notifications. No locking
+	 * between setting and use - so if userspace updates this
+	 * while child cgroups exist, you could miss a
+	 * notification. We ensure that it's always a valid
+	 * NUL-terminated string */
+	char release_agent_path[PATH_MAX];
 };
 
 
@@ -110,7 +120,13 @@ static int need_forkexit_callback;
 
 /* bits in struct cgroup flags field */
 enum {
+	/* Control Group is dead */
 	CONT_REMOVED,
+	/* Control Group has previously had a child cgroup or a task,
+	 * but no longer (only if CONT_NOTIFY_ON_RELEASE is set) */
+	CONT_RELEASABLE,
+	/* Control Group requires release notifications to userspace */
+	CONT_NOTIFY_ON_RELEASE,
 };
 
 /* convenient tests for these bits */
@@ -124,6 +140,19 @@ enum {
 	ROOT_NOPREFIX, /* mounted subsystems have no named prefix */
 };
 
+inline int cgroup_is_releasable(const struct cgroup *cont)
+{
+	const int bits =
+		(1 << CONT_RELEASABLE) |
+		(1 << CONT_NOTIFY_ON_RELEASE);
+	return (cont->flags & bits) == bits;
+}
+
+inline int notify_on_release(const struct cgroup *cont)
+{
+	return test_bit(CONT_NOTIFY_ON_RELEASE, &cont->flags);
+}
+
 /*
  * for_each_subsys() allows you to iterate on each subsystem attached to
  * an active hierarchy
@@ -135,6 +164,14 @@ list_for_each_entry(_ss, &_root->subsys_list, sibling)
 #define for_each_root(_root) \
 list_for_each_entry(_root, &roots, root_list)
 
+/* the list of cgroups eligible for automatic release. Protected by
+ * release_list_lock */
+static LIST_HEAD(release_list);
+static DEFINE_SPINLOCK(release_list_lock);
+static void cgroup_release_agent(struct work_struct *work);
+static DECLARE_WORK(release_agent_work, cgroup_release_agent);
+static void check_for_release(struct cgroup *cont);
+
 /* Link structure for associating css_set objects with cgroups */
 struct cg_cgroup_link {
 	/*
@@ -189,11 +226,8 @@ static int use_task_css_set_links;
 /*
  * unlink a css_set from the list and free it
  */
-static void release_css_set(struct kref *k)
+static void unlink_css_set(struct css_set *cg)
 {
-	struct css_set *cg = container_of(k, struct css_set, ref);
-	int i;
-
 	write_lock(&css_set_lock);
 	list_del(&cg->list);
 	css_set_count--;
@@ -206,11 +240,39 @@ static void release_css_set(struct kref *k)
 		kfree(link);
 	}
 	write_unlock(&css_set_lock);
-	for (i = 0; i < CGROUP_SUBSYS_COUNT; i++)
-		atomic_dec(&cg->subsys[i]->cgroup->count);
+}
+
+static void __release_css_set(struct kref *k, int taskexit)
+{
+	int i;
+	struct css_set *cg = container_of(k, struct css_set, ref);
+
+	unlink_css_set(cg);
+
+	rcu_read_lock();
+	for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
+		struct cgroup *cont = cg->subsys[i]->cgroup;
+		if (atomic_dec_and_test(&cont->count) &&
+		    notify_on_release(cont)) {
+			if (taskexit)
+				set_bit(CONT_RELEASABLE, &cont->flags);
+			check_for_release(cont);
+		}
+	}
+	rcu_read_unlock();
 	kfree(cg);
 }
 
+static void release_css_set(struct kref *k)
+{
+	__release_css_set(k, 0);
+}
+
+static void release_css_set_taskexit(struct kref *k)
+{
+	__release_css_set(k, 1);
+}
+
 /*
  * refcounted get/put for css_set objects
  */
@@ -224,6 +286,11 @@ static inline void put_css_set(struct css_set *cg)
 	kref_put(&cg->ref, release_css_set);
 }
 
+static inline void put_css_set_taskexit(struct css_set *cg)
+{
+	kref_put(&cg->ref, release_css_set_taskexit);
+}
+
 /*
  * find_existing_css_set() is a helper for
  * find_css_set(), and checks to see whether an existing
@@ -465,8 +532,6 @@ static struct css_set *find_css_set(
  * update of a tasks cgroup pointer by attach_task()
  */
 
-static DEFINE_MUTEX(cgroup_mutex);
-
 /**
  * cgroup_lock - lock out any changes to cgroup structures
  *
@@ -526,6 +591,13 @@ static void cgroup_diput(struct dentry *dentry, struct inode *inode)
 	if (S_ISDIR(inode->i_mode)) {
 		struct cgroup *cont = dentry->d_fsdata;
 		BUG_ON(!(cgroup_is_removed(cont)));
+		/* It's possible for external users to be holding css
+		 * reference counts on a cgroup; css_put() needs to
+		 * be able to access the cgroup after decrementing
+		 * the reference count in order to know if it needs to
+		 * queue the cgroup to be handled by the release
+		 * agent */
+		synchronize_rcu();
 		kfree(cont);
 	}
 	iput(inode);
@@ -657,6 +729,8 @@ static int cgroup_show_options(struct seq_file *seq, struct vfsmount *vfs)
 		seq_printf(seq, ",%s", ss->name);
 	if (test_bit(ROOT_NOPREFIX, &root->flags))
 		seq_puts(seq, ",noprefix");
+	if (strlen(root->release_agent_path))
+		seq_printf(seq, ",release_agent=%s", root->release_agent_path);
 	mutex_unlock(&cgroup_mutex);
 	return 0;
 }
@@ -664,6 +738,7 @@ static int cgroup_show_options(struct seq_file *seq, struct vfsmount *vfs)
 struct cgroup_sb_opts {
 	unsigned long subsys_bits;
 	unsigned long flags;
+	char *release_agent;
 };
 
 /* Convert a hierarchy specifier into a bitmask of subsystems and
@@ -675,6 +750,7 @@ static int parse_cgroupfs_options(char *data,
 
 	opts->subsys_bits = 0;
 	opts->flags = 0;
+	opts->release_agent = NULL;
 
 	while ((token = strsep(&o, ",")) != NULL) {
 		if (!*token)
@@ -683,6 +759,15 @@ static int parse_cgroupfs_options(char *data,
 			opts->subsys_bits = (1 << CGROUP_SUBSYS_COUNT) - 1;
 		} else if (!strcmp(token, "noprefix")) {
 			set_bit(ROOT_NOPREFIX, &opts->flags);
+		} else if (!strncmp(token, "release_agent=", 14)) {
+			/* Specifying two release agents is forbidden */
+			if (opts->release_agent)
+				return -EINVAL;
+			opts->release_agent = kzalloc(PATH_MAX, GFP_KERNEL);
+			if (!opts->release_agent)
+				return -ENOMEM;
+			strncpy(opts->release_agent, token + 14, PATH_MAX - 1);
+			opts->release_agent[PATH_MAX - 1] = 0;
 		} else {
 			struct cgroup_subsys *ss;
 			int i;
@@ -732,7 +817,11 @@ static int cgroup_remount(struct super_block *sb, int *flags, char *data)
 	if (!ret)
 		cgroup_populate_dir(cont);
 
+	if (opts.release_agent)
+		strcpy(root->release_agent_path, opts.release_agent);
  out_unlock:
+	if (opts.release_agent)
+		kfree(opts.release_agent);
 	mutex_unlock(&cgroup_mutex);
 	mutex_unlock(&cont->dentry->d_inode->i_mutex);
 	return ret;
@@ -756,6 +845,7 @@ static void init_cgroup_root(struct cgroupfs_root *root)
 	INIT_LIST_HEAD(&cont->sibling);
 	INIT_LIST_HEAD(&cont->children);
 	INIT_LIST_HEAD(&cont->css_sets);
+	INIT_LIST_HEAD(&cont->release_list);
 }
 
 static int cgroup_test_super(struct super_block *sb, void *data)
@@ -830,8 +920,11 @@ static int cgroup_get_sb(struct file_system_type *fs_type,
 
 	/* First find the desired set of subsystems */
 	ret = parse_cgroupfs_options(data, &opts);
-	if (ret)
+	if (ret) {
+		if (opts.release_agent)
+			kfree(opts.release_agent);
 		return ret;
+	}
 
 	root = kzalloc(sizeof(*root), GFP_KERNEL);
 	if (!root)
@@ -840,6 +933,10 @@ static int cgroup_get_sb(struct file_system_type *fs_type,
 	init_cgroup_root(root);
 	root->subsys_bits = opts.subsys_bits;
 	root->flags = opts.flags;
+	if (opts.release_agent) {
+		strcpy(root->release_agent_path, opts.release_agent);
+		kfree(opts.release_agent);
+	}
 
 	sb = sget(fs_type, cgroup_test_super, cgroup_set_super, root);
 
@@ -1120,7 +1217,7 @@ static int attach_task(struct cgroup *cont, struct task_struct *tsk)
 			ss->attach(ss, cont, oldcont, tsk);
 		}
 	}
-
+	set_bit(CONT_RELEASABLE, &oldcont->flags);
 	synchronize_rcu();
 	put_css_set(cg);
 	return 0;
@@ -1170,6 +1267,9 @@ enum cgroup_filetype {
 	FILE_ROOT,
 	FILE_DIR,
 	FILE_TASKLIST,
+	FILE_NOTIFY_ON_RELEASE,
+	FILE_RELEASABLE,
+	FILE_RELEASE_AGENT,
 };
 
 static ssize_t cgroup_write_uint(struct cgroup *cont, struct cftype *cft,
@@ -1240,6 +1340,32 @@ static ssize_t cgroup_common_file_write(struct cgroup *cont,
 	case FILE_TASKLIST:
 		retval = attach_task_by_pid(cont, buffer);
 		break;
+	case FILE_NOTIFY_ON_RELEASE:
+		clear_bit(CONT_RELEASABLE, &cont->flags);
+		if (simple_strtoul(buffer, NULL, 10) != 0)
+			set_bit(CONT_NOTIFY_ON_RELEASE, &cont->flags);
+		else
+			clear_bit(CONT_NOTIFY_ON_RELEASE, &cont->flags);
+		break;
+	case FILE_RELEASE_AGENT:
+	{
+		struct cgroupfs_root *root = cont->root;
+		/* Strip trailing newline */
+		if (nbytes && (buffer[nbytes-1] == '\n')) {
+			buffer[nbytes-1] = 0;
+		}
+		if (nbytes < sizeof(root->release_agent_path)) {
+			/* We never write anything other than '\0'
+			 * into the last char of release_agent_path,
+			 * so it always remains a NUL-terminated
+			 * string */
+			strncpy(root->release_agent_path, buffer, nbytes);
+			root->release_agent_path[nbytes] = 0;
+		} else {
+			retval = -ENOSPC;
+		}
+		break;
+	}
 	default:
 		retval = -EINVAL;
 		goto out2;
@@ -1281,6 +1407,49 @@ static ssize_t cgroup_read_uint(struct cgroup *cont, struct cftype *cft,
 	return simple_read_from_buffer(buf, nbytes, ppos, tmp, len);
 }
 
+static ssize_t cgroup_common_file_read(struct cgroup *cont,
+					  struct cftype *cft,
+					  struct file *file,
+					  char __user *buf,
+					  size_t nbytes, loff_t *ppos)
+{
+	enum cgroup_filetype type = cft->private;
+	char *page;
+	ssize_t retval = 0;
+	char *s;
+
+	if (!(page = (char *)__get_free_page(GFP_KERNEL)))
+		return -ENOMEM;
+
+	s = page;
+
+	switch (type) {
+	case FILE_RELEASE_AGENT:
+	{
+		struct cgroupfs_root *root;
+		size_t n;
+		mutex_lock(&cgroup_mutex);
+		root = cont->root;
+		n = strnlen(root->release_agent_path,
+			    sizeof(root->release_agent_path));
+		n = min(n, (size_t) PAGE_SIZE);
+		strncpy(s, root->release_agent_path, n);
+		mutex_unlock(&cgroup_mutex);
+		s += n;
+		break;
+	}
+	default:
+		retval = -EINVAL;
+		goto out;
+	}
+	*s++ = '\n';
+
+	retval = simple_read_from_buffer(buf, nbytes, ppos, page, s - page);
+out:
+	free_page((unsigned long)page);
+	return retval;
+}
+
 static ssize_t cgroup_file_read(struct file *file, char __user *buf,
 				   size_t nbytes, loff_t *ppos)
 {
@@ -1699,16 +1868,49 @@ static int cgroup_tasks_release(struct inode *unused_inode,
 	return 0;
 }
 
+static u64 cgroup_read_notify_on_release(struct cgroup *cont,
+					    struct cftype *cft)
+{
+	return notify_on_release(cont);
+}
+
+static u64 cgroup_read_releasable(struct cgroup *cont, struct cftype *cft)
+{
+	return test_bit(CONT_RELEASABLE, &cont->flags);
+}
+
 /*
  * for the common functions, 'private' gives the type of file
  */
-static struct cftype cft_tasks = {
-	.name = "tasks",
-	.open = cgroup_tasks_open,
-	.read = cgroup_tasks_read,
+static struct cftype files[] = {
+	{
+		.name = "tasks",
+		.open = cgroup_tasks_open,
+		.read = cgroup_tasks_read,
+		.write = cgroup_common_file_write,
+		.release = cgroup_tasks_release,
+		.private = FILE_TASKLIST,
+	},
+
+	{
+		.name = "notify_on_release",
+		.read_uint = cgroup_read_notify_on_release,
+		.write = cgroup_common_file_write,
+		.private = FILE_NOTIFY_ON_RELEASE,
+	},
+
+	{
+		.name = "releasable",
+		.read_uint = cgroup_read_releasable,
+		.private = FILE_RELEASABLE,
+	}
+};
+
+static struct cftype cft_release_agent = {
+	.name = "release_agent",
+	.read = cgroup_common_file_read,
 	.write = cgroup_common_file_write,
-	.release = cgroup_tasks_release,
-	.private = FILE_TASKLIST,
+	.private = FILE_RELEASE_AGENT,
 };
 
 static int cgroup_populate_dir(struct cgroup *cont)
@@ -1719,10 +1921,15 @@ static int cgroup_populate_dir(struct cgroup *cont)
 	/* First clear out any existing files */
 	cgroup_clear_directory(cont->dentry);
 
-	err = cgroup_add_file(cont, NULL, &cft_tasks);
+	err = cgroup_add_files(cont, NULL, files, ARRAY_SIZE(files));
 	if (err < 0)
 		return err;
 
+	if (cont == cont->top_cgroup) {
+		if ((err = cgroup_add_file(cont, NULL, &cft_release_agent)) < 0)
+			return err;
+	}
+
 	for_each_subsys(cont->root, ss) {
 		if (ss->populate && (err = ss->populate(ss, cont)) < 0)
 			return err;
@@ -1779,6 +1986,7 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
 	INIT_LIST_HEAD(&cont->sibling);
 	INIT_LIST_HEAD(&cont->children);
 	INIT_LIST_HEAD(&cont->css_sets);
+	INIT_LIST_HEAD(&cont->release_list);
 
 	cont->parent = parent;
 	cont->root = parent->root;
@@ -1840,6 +2048,38 @@ static int cgroup_mkdir(struct inode *dir, struct dentry *dentry, int mode)
 	return cgroup_create(c_parent, dentry, mode | S_IFDIR);
 }
 
+static inline int cgroup_has_css_refs(struct cgroup *cont)
+{
+	/* Check the reference count on each subsystem. Since we
+	 * already established that there are no tasks in the
+	 * cgroup, if the css refcount is also 0, then there should
+	 * be no outstanding references, so the subsystem is safe to
+	 * destroy. We scan across all subsystems rather than using
+	 * the per-hierarchy linked list of mounted subsystems since
+	 * we can be called via check_for_release() with no
+	 * synchronization other than RCU, and the subsystem linked
+	 * list isn't RCU-safe */
+	int i;
+	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 != cont->root)
+			continue;
+		css = cont->subsys[ss->subsys_id];
+		/* When called from check_for_release() it's possible
+		 * that by this point the cgroup has been removed
+		 * and the css deleted. But a false-positive doesn't
+		 * matter, since it can only happen if the cgroup
+		 * has been deleted and hence no longer needs the
+		 * release agent to be called anyway. */
+		if (css && atomic_read(&css->refcnt)) {
+			return 1;
+		}
+	}
+	return 0;
+}
+
 static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
 {
 	struct cgroup *cont = dentry->d_fsdata;
@@ -1848,7 +2088,6 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
 	struct cgroup_subsys *ss;
 	struct super_block *sb;
 	struct cgroupfs_root *root;
-	int css_busy = 0;
 
 	/* the vfs holds both inode->i_mutex already */
 
@@ -1866,20 +2105,7 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
 	root = cont->root;
 	sb = root->sb;
 
-	/* Check the reference count on each subsystem. Since we
-	 * already established that there are no tasks in the
-	 * cgroup, if the css refcount is also 0, then there should
-	 * be no outstanding references, so the subsystem is safe to
-	 * destroy */
-	for_each_subsys(root, ss) {
-		struct cgroup_subsys_state *css;
-		css = cont->subsys[ss->subsys_id];
-		if (atomic_read(&css->refcnt)) {
-			css_busy = 1;
-			break;
-		}
-	}
-	if (css_busy) {
+	if (cgroup_has_css_refs(cont)) {
 		mutex_unlock(&cgroup_mutex);
 		return -EBUSY;
 	}
@@ -1889,7 +2115,11 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
 			ss->destroy(ss, cont);
 	}
 
+	spin_lock(&release_list_lock);
 	set_bit(CONT_REMOVED, &cont->flags);
+	if (!list_empty(&cont->release_list))
+		list_del(&cont->release_list);
+	spin_unlock(&release_list_lock);
 	/* delete my sibling from parent->children */
 	list_del(&cont->sibling);
 	spin_lock(&cont->dentry->d_lock);
@@ -1901,6 +2131,9 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
 	dput(d);
 	root->number_of_cgroups--;
 
+	set_bit(CONT_RELEASABLE, &parent->flags);
+	check_for_release(parent);
+
 	mutex_unlock(&cgroup_mutex);
 	/* Drop the active superblock reference that we took when we
 	 * created the cgroup */
@@ -1938,15 +2171,15 @@ static void cgroup_init_subsys(struct cgroup_subsys *ss)
  	/* If this subsystem requested that it be notified with fork
  	 * events, we should send it one now for every process in the
  	 * system */
- 	if (ss->fork) {
- 		struct task_struct *g, *p;
-
- 		read_lock(&tasklist_lock);
- 		do_each_thread(g, p) {
- 			ss->fork(ss, p);
- 		} while_each_thread(g, p);
- 		read_unlock(&tasklist_lock);
- 	}
+	if (ss->fork) {
+		struct task_struct *g, *p;
+
+		read_lock(&tasklist_lock);
+		do_each_thread(g, p) {
+			ss->fork(ss, p);
+		} while_each_thread(g, p);
+		read_unlock(&tasklist_lock);
+	}
 
 	need_forkexit_callback |= ss->fork || ss->exit;
 
@@ -2263,7 +2496,7 @@ void cgroup_exit(struct task_struct *tsk, int run_callbacks)
 	tsk->cgroups = &init_css_set;
 	task_unlock(tsk);
 	if (cg)
-		put_css_set(cg);
+		put_css_set_taskexit(cg);
 }
 
 /**
@@ -2374,7 +2607,10 @@ int cgroup_clone(struct task_struct *tsk, struct cgroup_subsys *subsys)
 
  out_release:
 	mutex_unlock(&inode->i_mutex);
+
+	mutex_lock(&cgroup_mutex);
 	put_css_set(cg);
+	mutex_unlock(&cgroup_mutex);
 	deactivate_super(parent->root->sb);
 	return ret;
 }
@@ -2404,3 +2640,111 @@ int cgroup_is_descendant(const struct cgroup *cont)
 	ret = (cont == target);
 	return ret;
 }
+
+static void check_for_release(struct cgroup *cont)
+{
+	/* All of these checks rely on RCU to keep the cgroup
+	 * structure alive */
+	if (cgroup_is_releasable(cont) && !atomic_read(&cont->count)
+	    && list_empty(&cont->children) && !cgroup_has_css_refs(cont)) {
+		/* Control Group is currently removeable. If it's not
+		 * already queued for a userspace notification, queue
+		 * it now */
+		int need_schedule_work = 0;
+		spin_lock(&release_list_lock);
+		if (!cgroup_is_removed(cont) &&
+		    list_empty(&cont->release_list)) {
+			list_add(&cont->release_list, &release_list);
+			need_schedule_work = 1;
+		}
+		spin_unlock(&release_list_lock);
+		if (need_schedule_work)
+			schedule_work(&release_agent_work);
+	}
+}
+
+void __css_put(struct cgroup_subsys_state *css)
+{
+	struct cgroup *cont = css->cgroup;
+	rcu_read_lock();
+	if (atomic_dec_and_test(&css->refcnt) && notify_on_release(cont)) {
+		set_bit(CONT_RELEASABLE, &cont->flags);
+		check_for_release(cont);
+	}
+	rcu_read_unlock();
+}
+
+/*
+ * Notify userspace when a cgroup is released, by running the
+ * configured release agent with the name of the cgroup (path
+ * relative to the root of cgroup file system) as the argument.
+ *
+ * Most likely, this user command will try to rmdir this cgroup.
+ *
+ * This races with the possibility that some other task will be
+ * attached to this cgroup before it is removed, or that some other
+ * user task will 'mkdir' a child cgroup of this cgroup.  That's ok.
+ * The presumed 'rmdir' will fail quietly if this cgroup is no longer
+ * unused, and this cgroup will be reprieved from its death sentence,
+ * to continue to serve a useful existence.  Next time it's released,
+ * we will get notified again, if it still has 'notify_on_release' set.
+ *
+ * The final arg to call_usermodehelper() is UMH_WAIT_EXEC, which
+ * means only wait until the task is successfully execve()'d.  The
+ * separate release agent task is forked by call_usermodehelper(),
+ * then control in this thread returns here, without waiting for the
+ * release agent task.  We don't bother to wait because the caller of
+ * this routine has no use for the exit status of the release agent
+ * task, so no sense holding our caller up for that.
+ *
+ */
+
+static void cgroup_release_agent(struct work_struct *work)
+{
+	BUG_ON(work != &release_agent_work);
+	mutex_lock(&cgroup_mutex);
+	spin_lock(&release_list_lock);
+	while (!list_empty(&release_list)) {
+		char *argv[3], *envp[3];
+		int i;
+		char *pathbuf;
+		struct cgroup *cont = list_entry(release_list.next,
+						    struct cgroup,
+						    release_list);
+		list_del_init(&cont->release_list);
+		spin_unlock(&release_list_lock);
+		pathbuf = kmalloc(PAGE_SIZE, GFP_KERNEL);
+		if (!pathbuf) {
+			spin_lock(&release_list_lock);
+			continue;
+		}
+
+		if (cgroup_path(cont, pathbuf, PAGE_SIZE) < 0) {
+			kfree(pathbuf);
+			spin_lock(&release_list_lock);
+			continue;
+		}
+
+		i = 0;
+		argv[i++] = cont->root->release_agent_path;
+		argv[i++] = (char *)pathbuf;
+		argv[i] = NULL;
+
+		i = 0;
+		/* minimal command environment */
+		envp[i++] = "HOME=/";
+		envp[i++] = "PATH=/sbin:/bin:/usr/sbin:/usr/bin";
+		envp[i] = NULL;
+
+		/* Drop the lock while we invoke the usermode helper,
+		 * since the exec could involve hitting disk and hence
+		 * be a slow process */
+		mutex_unlock(&cgroup_mutex);
+		call_usermodehelper(argv[0], argv, envp, UMH_WAIT_EXEC);
+		kfree(pathbuf);
+		mutex_lock(&cgroup_mutex);
+		spin_lock(&release_list_lock);
+	}
+	spin_unlock(&release_list_lock);
+	mutex_unlock(&cgroup_mutex);
+}