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	 <ca59c422b495907678915db636f70a8d029cbf3a.1260571675.git.kirill@shutemov.name>
Date: Tue, 15 Dec 2009 11:11:16 +0200
Message-ID: <cc557aab0912150111k41517b41t8999568db3bd8daa@mail.gmail.com>
Subject: Re: [PATCH RFC v2 1/4] cgroup: implement eventfd-based generic API 
	for notifications
From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Paul Menage <menage@google.com>, Li Zefan <lizf@cn.fujitsu.com>,
       containers@lists.linux-foundation.org
Cc: Andrew Morton <akpm@linux-foundation.org>,
       KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
       Balbir Singh <balbir@linux.vnet.ibm.com>,
       Pavel Emelyanov <xemul@openvz.org>, Dan Malek <dan@embeddedalley.com>,
       Vladislav Buzov <vbuzov@embeddedalley.com>,
       Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>,
       linux-kernel@vger.kernel.org,
       "Kirill A. Shutemov" <kirill@shutemov.name>, linux-mm@kvack.org
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Could anybody review the patch?

Thank you.

On Sat, Dec 12, 2009 at 12:59 AM, Kirill A. Shutemov
<kirill@shutemov.name> 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
> + *  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.
> + */
> +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;
> +};
> +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);
> +       }
> +       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);
> +       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
> +                * cgroup_event_remove() may sleep, so we have
> +                * to run it in a 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);
> +}
> +
> +static int cgroup_write_event_control(struct cgroup *cont, 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 = 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);
> +
> +       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,
> +       },
>  };
>
>  static struct cftype cft_release_agent = {
> --
> 1.6.5.3
>
>
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