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>

1 files changed, 386 insertions, 42 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 883928c0e147..d65a1246829f 100644
--- a/kernel/cgroup.c
+++ b/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);
+}