Re: [PATCH RFC v2 1/4] cgroup: implement eventfd-based generic API for notifications

[Li Zefan <lizf@cn.fujitsu.com>]

Kirill A. Shutemov wrote:
> This patch introduces write-only file "cgroup.event_control" in every
> cgroup.
> 
> To register new notification handler you need:
> - create an eventfd;
> - open a control file to be monitored. Callbacks register_event() and
>   unregister_event() must be defined for the control file;
> - write "<event_fd> <control_fd> <args>" to cgroup.event_control.
>   Interpretation of args is defined by control file implementation;
> 
> eventfd will be woken up by control file implementation or when the
> cgroup is removed.
> 
> To unregister notification handler just close eventfd.
> 
> If you need notification functionality for a control file you have to
> implement callbacks register_event() and unregister_event() in the
> struct cftype.
> 
> Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
> ---
>  include/linux/cgroup.h |   20 +++++
>  kernel/cgroup.c        |  215 +++++++++++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 234 insertions(+), 1 deletions(-)
> 
> diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
> index 0008dee..7ad3078 100644
> --- a/include/linux/cgroup.h
> +++ b/include/linux/cgroup.h
> @@ -220,6 +220,10 @@ struct cgroup {
>  
>  	/* For RCU-protected deletion */
>  	struct rcu_head rcu_head;
> +
> +	/* List of events which userspace want to recieve */
> +	struct list_head event_list;
> +	struct mutex event_list_mutex;
>  };
>  
>  /*
> @@ -362,6 +366,22 @@ struct cftype {
>  	int (*trigger)(struct cgroup *cgrp, unsigned int event);
>  
>  	int (*release)(struct inode *inode, struct file *file);
> +
> +	/*
> +	 * register_event() callback will be used to add new userspace
> +	 * waiter for changes related to the cftype. Implement it if
> +	 * you want to provide this functionality. Use eventfd_signal()
> +	 * on eventfd to send notification to userspace.
> +	 */
> +	int (*register_event)(struct cgroup *cgrp, struct cftype *cft,
> +			struct eventfd_ctx *eventfd, const char *args);
> +	/*
> +	 * unregister_event() callback will be called when userspace
> +	 * close the eventfd. This callback must be implemented, if you
> +	 * provide register_event().
> +	 */
> +	int (*unregister_event)(struct cgroup *cgrp, struct cftype *cft,
> +			struct eventfd_ctx *eventfd);
>  };
>  
>  struct cgroup_scanner {
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 0249f4b..f7ec3ca 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
>   *
> + *  Notifiactions support

s/Notifiactions/Notifications

> + *  Copyright (C) 2009 Nokia Corporation
> + *  Author: Kirill A. Shutemov
> + *
>   *  Copyright notices from the original cpuset code:
>   *  --------------------------------------------------
>   *  Copyright (C) 2003 BULL SA.
> @@ -51,6 +55,8 @@
>  #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>
>  
> @@ -146,6 +152,36 @@ struct css_id {
>  	unsigned short stack[0]; /* Array of Length (depth+1) */
>  };
>  
> +/*
> + * cgroup_event represents event which userspace want to recieve.

s/event/events ?

> + */
> +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;
> +};

Please add a blank line here.

> +static void cgroup_event_remove(struct cgroup_event *event);
>  
>  /* The list of hierarchy roots */
>  
> @@ -734,14 +770,29 @@ static struct inode *cgroup_new_inode(mode_t mode, struct super_block *sb)
>  static int cgroup_call_pre_destroy(struct cgroup *cgrp)
>  {
>  	struct cgroup_subsys *ss;
> +	struct cgroup_event *event, *tmp;
>  	int ret = 0;
>  
>  	for_each_subsys(cgrp->root, ss)
>  		if (ss->pre_destroy) {
>  			ret = ss->pre_destroy(ss, cgrp);
>  			if (ret)
> -				break;
> +				goto out;
>  		}
> +
> +	/*
> +	 * Unregister events and notify userspace.
> +	 * FIXME: How to avoid race with cgroup_event_remove_work()
> +	 *        which runs from workqueue?
> +	 */
> +	mutex_lock(&cgrp->event_list_mutex);
> +	list_for_each_entry_safe(event, tmp, &cgrp->event_list, list) {
> +		cgroup_event_remove(event);
> +		eventfd_signal(event->eventfd, 1);

How can you access event after you kfree()ed it in cgroup_event_remove()?

> +	}
> +	mutex_unlock(&cgrp->event_list_mutex);
> +
> +out:
>  	return ret;
>  }
>  
> @@ -1136,6 +1187,8 @@ 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);
> +	mutex_init(&cgrp->event_list_mutex);
>  }
>  
>  static void init_cgroup_root(struct cgroupfs_root *root)
> @@ -1935,6 +1988,16 @@ 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);

I don't think this check is needed.

> +	return __d_cft(file->f_dentry);
> +}
> +
>  static int cgroup_create_file(struct dentry *dentry, mode_t mode,
>  				struct super_block *sb)
>  {
> @@ -2789,6 +2852,151 @@ static int cgroup_write_notify_on_release(struct cgroup *cgrp,
>  	return 0;
>  }
>  
> +static inline void cgroup_event_remove(struct cgroup_event *event)
> +{
> +	struct cgroup *cgrp = event->cgrp;
> +
> +	BUG_ON(event->cft->unregister_event(cgrp, event->cft, event->eventfd));
> +	eventfd_ctx_put(event->eventfd);
> +	remove_wait_queue(event->wqh, &event->wait);
> +	list_del(&event->list);
> +	kfree(event);
> +}
> +
> +static void cgroup_event_remove_work(struct work_struct *work)
> +{
> +	struct cgroup_event *event = container_of(work, struct cgroup_event,
> +			remove);
> +	struct cgroup *cgrp = event->cgrp;
> +
> +	mutex_lock(&cgrp->event_list_mutex);
> +	cgroup_event_remove(event);
> +	mutex_unlock(&cgrp->event_list_mutex);
> +}
> +
> +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);
> +	unsigned long flags = (unsigned long)key;
> +
> +	if (flags & POLLHUP)
> +		/*
> +		 * This function called with spinlock taken, but

s/called/is called/ ?

> +		 * cgroup_event_remove() may sleep, so we have
> +		 * to run it in a workqueue.
> +		 */
> +		schedule_work(&event->remove);

Please use:

	if (...) {
		...
	}

> +
> +	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);
> +}
> +
> +static int cgroup_write_event_control(struct cgroup *cont, struct cftype *cft,

Please consistently use "cgrp"

> +				      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 = cont;
> +	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_work);
> +
> +	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(cont, event->cft,
> +			event->eventfd, buffer);
> +	if (ret)
> +		goto fail;
> +
> +	efile->f_op->poll(efile, &event->pt);
> +
> +	mutex_lock(&cont->event_list_mutex);
> +	list_add(&event->list, &cont->event_list);
> +	mutex_unlock(&cont->event_list_mutex);
> +
> +	fput(cfile);
> +	fput(efile);
> +
> +	return 0;
> +
> +fail:
> +	if (!IS_ERR(cfile))
> +		fput(cfile);
> +
> +	if (event && event->eventfd && !IS_ERR(event->eventfd))
> +		eventfd_ctx_put(event->eventfd);
> +
> +	if (!IS_ERR(efile))
> +		fput(efile);
> +
> +	if (event)
> +		kfree(event);

kfree(NULL) is ok

> +
> +	return ret;
> +}
> +
>  /*
>   * for the common functions, 'private' gives the type of file
>   */
> @@ -2814,6 +3022,11 @@ 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,

We want this file to be writable to everyone ?

> +	},
>  };
>  
>  static struct cftype cft_release_agent = {