Message-ID: <20070723174102.GA22624@sergelap.austin.ibm.com>
Date:	Mon, 23 Jul 2007 12:41:02 -0500
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	menage@...gle.com
Cc:	akpm@...ux-foundation.org, dev@...ru, xemul@...ru,
	serue@...ibm.com, vatsa@...ibm.com, ebiederm@...ssion.com,
	haveblue@...ibm.com, svaidy@...ux.vnet.ibm.com, balbir@...ibm.com,
	pj@....com, ckrm-tech@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org, rohitseth@...gle.com,
	mbligh@...gle.com, containers@...ts.osdl.org, devel@...nvz.org
Subject: Re: [PATCH 07/10] Task Containers(V11):  Automatic userspace notification of idle containers

Quoting menage@...gle.com (menage@...gle.com):
> This patch adds the following files to the container filesystem:
> 
> notify_on_release - configures/reports whether the container subsystem should
> attempt to run a release script when this container 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 container 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; containers that need to have their release agents invoked by
> the workqueue task are linked on to a list.


Hi Paul,

my tree may be a bit crufty, but I had to #include <linux/kmod.h> in
order for this to compile on s390.

thanks,
-serge

> Signed-off-by: Paul Menage <menage@...gle.com>
> ---
> 
>  include/linux/container.h |   11 -
>  kernel/container.c        |  425 +++++++++++++++++++++++++++++++++++++++++-----
>  2 files changed, 393 insertions(+), 43 deletions(-)
> 
> Index: container-2.6.22-rc6-mm1/include/linux/container.h
> ===================================================================
> --- container-2.6.22-rc6-mm1.orig/include/linux/container.h
> +++ container-2.6.22-rc6-mm1/include/linux/container.h
> @@ -77,10 +77,11 @@ static inline void css_get(struct contai
>   * css_get()
>   */
> 
> +extern void __css_put(struct container_subsys_state *css);
>  static inline void css_put(struct container_subsys_state *css)
>  {
>  	if (!test_bit(CSS_ROOT, &css->flags))
> -		atomic_dec(&css->refcnt);
> +		__css_put(css);
>  }
> 
>  struct container {
> @@ -112,6 +113,13 @@ struct container {
>  	 * tasks in this container. Protected by css_group_lock
>  	 */
>  	struct list_head css_groups;
> +
> +	/*
> +	 * Linked list running through all containers that can
> +	 * potentially be reaped by the release agent. Protected by
> +	 * release_list_lock
> +	 */
> +	struct list_head release_list;
>  };
> 
>  /* A css_group is a structure holding pointers to a set of
> @@ -285,7 +293,6 @@ struct task_struct *container_iter_next(
>  					struct container_iter *it);
>  void container_iter_end(struct container *cont, struct container_iter *it);
> 
> -
>  #else /* !CONFIG_CONTAINERS */
> 
>  static inline int container_init_early(void) { return 0; }
> Index: container-2.6.22-rc6-mm1/kernel/container.c
> ===================================================================
> --- container-2.6.22-rc6-mm1.orig/kernel/container.c
> +++ container-2.6.22-rc6-mm1/kernel/container.c
> @@ -44,6 +44,8 @@
>  #include <linux/sort.h>
>  #include <asm/atomic.h>
> 
> +static DEFINE_MUTEX(container_mutex);
> +
>  /* Generate an array of container subsystem pointers */
>  #define SUBSYS(_x) &_x ## _subsys,
> 
> @@ -82,6 +84,13 @@ struct containerfs_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 subcontainers exist, you could miss a
> +	 * notification. We ensure that it's always a valid
> +	 * NUL-terminated string */
> +	char release_agent_path[PATH_MAX];
>  };
> 
> 
> @@ -109,7 +118,13 @@ static int need_forkexit_callback;
> 
>  /* bits in struct container flags field */
>  enum {
> +	/* Container is dead */
>  	CONT_REMOVED,
> +	/* Container has previously had a child container or a task,
> +	 * but no longer (only if CONT_NOTIFY_ON_RELEASE is set) */
> +	CONT_RELEASABLE,
> +	/* Container requires release notifications to userspace */
> +	CONT_NOTIFY_ON_RELEASE,
>  };
> 
>  /* convenient tests for these bits */
> @@ -123,6 +138,19 @@ enum {
>  	ROOT_NOPREFIX, /* mounted subsystems have no named prefix */
>  };
> 
> +inline int container_is_releasable(const struct container *cont)
> +{
> +	const int bits =
> +		(1 << CONT_RELEASABLE) |
> +		(1 << CONT_NOTIFY_ON_RELEASE);
> +	return (cont->flags & bits) == bits;
> +}
> +
> +inline int notify_on_release(const struct container *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
> @@ -134,6 +162,14 @@ list_for_each_entry(_ss, &_root->subsys_
>  #define for_each_root(_root) \
>  list_for_each_entry(_root, &roots, root_list)
> 
> +/* the list of containers eligible for automatic release. Protected by
> + * release_list_lock */
> +static LIST_HEAD(release_list);
> +static DEFINE_SPINLOCK(release_list_lock);
> +static void container_release_agent(struct work_struct *work);
> +static DECLARE_WORK(release_agent_work, container_release_agent);
> +static void check_for_release(struct container *cont);
> +
>  /* Link structure for associating css_group objects with containers */
>  struct cg_container_link {
>  	/*
> @@ -188,11 +224,8 @@ static int use_task_css_group_links;
>  /*
>   * unlink a css_group from the list and free it
>   */
> -static void release_css_group(struct kref *k)
> +static void unlink_css_group(struct css_group *cg)
>  {
> -	struct css_group *cg = container_of(k, struct css_group, ref);
> -	int i;
> -
>  	write_lock(&css_group_lock);
>  	list_del(&cg->list);
>  	css_group_count--;
> @@ -205,11 +238,39 @@ static void release_css_group(struct kre
>  		kfree(link);
>  	}
>  	write_unlock(&css_group_lock);
> -	for (i = 0; i < CONTAINER_SUBSYS_COUNT; i++)
> -		atomic_dec(&cg->subsys[i]->container->count);
> +}
> +
> +static void __release_css_group(struct kref *k, int taskexit)
> +{
> +	int i;
> +	struct css_group *cg = container_of(k, struct css_group, ref);
> +
> +	unlink_css_group(cg);
> +
> +	rcu_read_lock();
> +	for (i = 0; i < CONTAINER_SUBSYS_COUNT; i++) {
> +		struct container *cont = cg->subsys[i]->container;
> +		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_group(struct kref *k)
> +{
> +	__release_css_group(k, 0);
> +}
> +
> +static void release_css_group_taskexit(struct kref *k)
> +{
> +	__release_css_group(k, 1);
> +}
> +
>  /*
>   * refcounted get/put for css_group objects
>   */
> @@ -223,6 +284,11 @@ static inline void put_css_group(struct 
>  	kref_put(&cg->ref, release_css_group);
>  }
> 
> +static inline void put_css_group_taskexit(struct css_group *cg)
> +{
> +	kref_put(&cg->ref, release_css_group_taskexit);
> +}
> +
>  /*
>   * find_existing_css_group() is a helper for
>   * find_css_group(), and checks to see whether an existing
> @@ -464,8 +530,6 @@ static struct css_group *find_css_group(
>   * update of a tasks container pointer by attach_task()
>   */
> 
> -static DEFINE_MUTEX(container_mutex);
> -
>  /**
>   * container_lock - lock out any changes to container structures
>   *
> @@ -524,6 +588,13 @@ static void container_diput(struct dentr
>  	if (S_ISDIR(inode->i_mode)) {
>  		struct container *cont = dentry->d_fsdata;
>  		BUG_ON(!(container_is_removed(cont)));
> +		/* It's possible for external users to be holding css
> +		 * reference counts on a container; css_put() needs to
> +		 * be able to access the container after decrementing
> +		 * the reference count in order to know if it needs to
> +		 * queue the container to be handled by the release
> +		 * agent */
> +		synchronize_rcu();
>  		kfree(cont);
>  	}
>  	iput(inode);
> @@ -668,6 +739,8 @@ static int container_show_options(struct
>  		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(&container_mutex);
>  	return 0;
>  }
> @@ -675,6 +748,7 @@ static int container_show_options(struct
>  struct container_sb_opts {
>  	unsigned long subsys_bits;
>  	unsigned long flags;
> +	char *release_agent;
>  };
> 
>  /* Convert a hierarchy specifier into a bitmask of subsystems and
> @@ -686,6 +760,7 @@ static int parse_containerfs_options(cha
> 
>  	opts->subsys_bits = 0;
>  	opts->flags = 0;
> +	opts->release_agent = NULL;
> 
>  	while ((token = strsep(&o, ",")) != NULL) {
>  		if (!*token)
> @@ -694,6 +769,15 @@ static int parse_containerfs_options(cha
>  			opts->subsys_bits = (1 << CONTAINER_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 container_subsys *ss;
>  			int i;
> @@ -743,7 +827,11 @@ static int container_remount(struct supe
>  	if (!ret)
>  		container_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(&container_mutex);
>  	mutex_unlock(&cont->dentry->d_inode->i_mutex);
>  	return ret;
> @@ -767,6 +855,7 @@ static void init_container_root(struct c
>  	INIT_LIST_HEAD(&cont->sibling);
>  	INIT_LIST_HEAD(&cont->children);
>  	INIT_LIST_HEAD(&cont->css_groups);
> +	INIT_LIST_HEAD(&cont->release_list);
>  }
> 
>  static int container_test_super(struct super_block *sb, void *data)
> @@ -841,8 +930,11 @@ static int container_get_sb(struct file_
> 
>  	/* First find the desired set of subsystems */
>  	ret = parse_containerfs_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)
> @@ -851,6 +943,10 @@ static int container_get_sb(struct file_
>  	init_container_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, container_test_super, container_set_super, root);
> 
> @@ -1125,7 +1221,7 @@ static int attach_task(struct container 
>  			ss->attach(ss, cont, oldcont, tsk);
>  		}
>  	}
> -
> +	set_bit(CONT_RELEASABLE, &oldcont->flags);
>  	synchronize_rcu();
>  	put_css_group(cg);
>  	return 0;
> @@ -1175,6 +1271,9 @@ enum container_filetype {
>  	FILE_ROOT,
>  	FILE_DIR,
>  	FILE_TASKLIST,
> +	FILE_NOTIFY_ON_RELEASE,
> +	FILE_RELEASABLE,
> +	FILE_RELEASE_AGENT,
>  };
> 
>  static ssize_t container_common_file_write(struct container *cont,
> @@ -1212,6 +1311,32 @@ static ssize_t container_common_file_wri
>  	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 containerfs_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;
> @@ -1252,6 +1377,49 @@ static ssize_t container_read_uint(struc
>  	return simple_read_from_buffer(buf, nbytes, ppos, tmp, len);
>  }
> 
> +static ssize_t container_common_file_read(struct container *cont,
> +					  struct cftype *cft,
> +					  struct file *file,
> +					  char __user *buf,
> +					  size_t nbytes, loff_t *ppos)
> +{
> +	enum container_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 containerfs_root *root;
> +		size_t n;
> +		mutex_lock(&container_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(&container_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 container_file_read(struct file *file, char __user *buf,
>  				   size_t nbytes, loff_t *ppos)
>  {
> @@ -1667,16 +1835,49 @@ static int container_tasks_release(struc
>  	return 0;
>  }
> 
> +static u64 container_read_notify_on_release(struct container *cont,
> +					    struct cftype *cft)
> +{
> +	return notify_on_release(cont);
> +}
> +
> +static u64 container_read_releasable(struct container *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 = container_tasks_open,
> -	.read = container_tasks_read,
> +static struct cftype files[] = {
> +	{
> +		.name = "tasks",
> +		.open = container_tasks_open,
> +		.read = container_tasks_read,
> +		.write = container_common_file_write,
> +		.release = container_tasks_release,
> +		.private = FILE_TASKLIST,
> +	},
> +
> +	{
> +		.name = "notify_on_release",
> +		.read_uint = container_read_notify_on_release,
> +		.write = container_common_file_write,
> +		.private = FILE_NOTIFY_ON_RELEASE,
> +	},
> +
> +	{
> +		.name = "releasable",
> +		.read_uint = container_read_releasable,
> +		.private = FILE_RELEASABLE,
> +	}
> +};
> +
> +static struct cftype cft_release_agent = {
> +	.name = "release_agent",
> +	.read = container_common_file_read,
>  	.write = container_common_file_write,
> -	.release = container_tasks_release,
> -	.private = FILE_TASKLIST,
> +	.private = FILE_RELEASE_AGENT,
>  };
> 
>  static int container_populate_dir(struct container *cont)
> @@ -1687,10 +1888,15 @@ static int container_populate_dir(struct
>  	/* First clear out any existing files */
>  	container_clear_directory(cont->dentry);
> 
> -	err = container_add_file(cont, NULL, &cft_tasks);
> +	err = container_add_files(cont, NULL, files, ARRAY_SIZE(files));
>  	if (err < 0)
>  		return err;
> 
> +	if (cont == cont->top_container) {
> +		if ((err = container_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;
> @@ -1747,6 +1953,7 @@ static long container_create(struct cont
>  	INIT_LIST_HEAD(&cont->sibling);
>  	INIT_LIST_HEAD(&cont->children);
>  	INIT_LIST_HEAD(&cont->css_groups);
> +	INIT_LIST_HEAD(&cont->release_list);
> 
>  	cont->parent = parent;
>  	cont->root = parent->root;
> @@ -1808,6 +2015,38 @@ static int container_mkdir(struct inode 
>  	return container_create(c_parent, dentry, mode | S_IFDIR);
>  }
> 
> +static inline int container_has_css_refs(struct container *cont)
> +{
> +	/* Check the reference count on each subsystem. Since we
> +	 * already established that there are no tasks in the
> +	 * container, 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 < CONTAINER_SUBSYS_COUNT; i++) {
> +		struct container_subsys *ss = subsys[i];
> +		struct container_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 container has been removed
> +		 * and the css deleted. But a false-positive doesn't
> +		 * matter, since it can only happen if the container
> +		 * 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 container_rmdir(struct inode *unused_dir, struct dentry *dentry)
>  {
>  	struct container *cont = dentry->d_fsdata;
> @@ -1816,7 +2055,6 @@ static int container_rmdir(struct inode 
>  	struct container_subsys *ss;
>  	struct super_block *sb;
>  	struct containerfs_root *root;
> -	int css_busy = 0;
> 
>  	/* the vfs holds both inode->i_mutex already */
> 
> @@ -1834,20 +2072,7 @@ static int container_rmdir(struct inode 
>  	root = cont->root;
>  	sb = root->sb;
> 
> -	/* Check the reference count on each subsystem. Since we
> -	 * already established that there are no tasks in the
> -	 * container, 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 container_subsys_state *css;
> -		css = cont->subsys[ss->subsys_id];
> -		if (atomic_read(&css->refcnt)) {
> -			css_busy = 1;
> -			break;
> -		}
> -	}
> -	if (css_busy) {
> +	if (container_has_css_refs(cont)) {
>  		mutex_unlock(&container_mutex);
>  		return -EBUSY;
>  	}
> @@ -1857,7 +2082,11 @@ static int container_rmdir(struct inode 
>  			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);
> @@ -1869,6 +2098,9 @@ static int container_rmdir(struct inode 
>  	dput(d);
>  	root->number_of_containers--;
> 
> +	set_bit(CONT_RELEASABLE, &parent->flags);
> +	check_for_release(parent);
> +
>  	mutex_unlock(&container_mutex);
>  	/* Drop the active superblock reference that we took when we
>  	 * created the container */
> @@ -1906,15 +2138,15 @@ static void container_init_subsys(struct
>   	/* 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;
> +	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);
> - 	}
> +		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;
> 
> @@ -2241,7 +2473,7 @@ void container_exit(struct task_struct *
>  	tsk->containers = &init_css_group;
>  	task_unlock(tsk);
>  	if (cg)
> -		put_css_group(cg);
> +		put_css_group_taskexit(cg);
>  }
> 
>  /**
> @@ -2352,7 +2584,10 @@ int container_clone(struct task_struct *
> 
>   out_release:
>  	mutex_unlock(&inode->i_mutex);
> +
> +	mutex_lock(&container_mutex);
>  	put_css_group(cg);
> +	mutex_unlock(&container_mutex);
>  	deactivate_super(parent->root->sb);
>  	return ret;
>  }
> @@ -2382,3 +2617,111 @@ int container_is_descendant(const struct
>  	ret = (cont == target);
>  	return ret;
>  }
> +
> +static void check_for_release(struct container *cont)
> +{
> +	/* All of these checks rely on RCU to keep the container
> +	 * structure alive */
> +	if (container_is_releasable(cont) && !atomic_read(&cont->count)
> +	    && list_empty(&cont->children) && !container_has_css_refs(cont)) {
> +		/* Container 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 (!container_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 container_subsys_state *css)
> +{
> +	struct container *cont = css->container;
> +	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 container is released, by running the
> + * configured release agent with the name of the container (path
> + * relative to the root of container file system) as the argument.
> + *
> + * Most likely, this user command will try to rmdir this container.
> + *
> + * This races with the possibility that some other task will be
> + * attached to this container before it is removed, or that some other
> + * user task will 'mkdir' a child container of this container.  That's ok.
> + * The presumed 'rmdir' will fail quietly if this container is no longer
> + * unused, and this container 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 container_release_agent(struct work_struct *work)
> +{
> +	BUG_ON(work != &release_agent_work);
> +	mutex_lock(&container_mutex);
> +	spin_lock(&release_list_lock);
> +	while (!list_empty(&release_list)) {
> +		char *argv[3], *envp[3];
> +		int i;
> +		char *pathbuf;
> +		struct container *cont = list_entry(release_list.next,
> +						    struct container,
> +						    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 (container_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(&container_mutex);
> +		call_usermodehelper(argv[0], argv, envp, UMH_WAIT_EXEC);
> +		kfree(pathbuf);
> +		mutex_lock(&container_mutex);
> +		spin_lock(&release_list_lock);
> +	}
> +	spin_unlock(&release_list_lock);
> +	mutex_unlock(&container_mutex);
> +}
> 
> --
-
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