cgroup: fix and update documentation

Misc fixes and updates, make the doc consistent with current cgroup
implementation.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: Li Zefan

Documentation/cgroups.txt

@@ -28,7 +28,7 @@ CONTENTS:
 4. Questions
 
 1. Control Groups
-==========
+=================
 
 1.1 What are cgroups ?
 ----------------------
@@ -143,10 +143,10 @@ proliferation of such cgroups.
 
 Also lets say that the administrator would like to give enhanced network
 access temporarily to a student's browser (since it is night and the user
-wants to do online gaming :)  OR give one of the students simulation
+wants to do online gaming :))  OR give one of the students simulation
 apps enhanced CPU power,
 
-With ability to write pids directly to resource classes, its just a
+With ability to write pids directly to resource classes, it's just a
 matter of :
 
        # echo pid > /mnt/network/<new_class>/tasks
@@ -227,10 +227,13 @@ Each cgroup is represented by a directory in the cgroup file system
 containing the following files describing that cgroup:
 
  - tasks: list of tasks (by pid) attached to that cgroup
- - notify_on_release flag: run /sbin/cgroup_release_agent on exit?
+ - releasable flag: cgroup currently removeable?
+ - notify_on_release flag: run the release agent on exit?
+ - release_agent: the path to use for release notifications (this file
+   exists in the top cgroup only)
 
 Other subsystems such as cpusets may add additional files in each
-cgroup dir
+cgroup dir.
 
 New cgroups are created using the mkdir system call or shell
 command.  The properties of a cgroup, such as its flags, are
@@ -257,7 +260,7 @@ performance.
 To allow access from a cgroup to the css_sets (and hence tasks)
 that comprise it, a set of cg_cgroup_link objects form a lattice;
 each cg_cgroup_link is linked into a list of cg_cgroup_links for
-a single cgroup on its cont_link_list field, and a list of
+a single cgroup on its cgrp_link_list field, and a list of
 cg_cgroup_links for a single css_set on its cg_link_list.
 
 Thus the set of tasks in a cgroup can be listed by iterating over
@@ -271,9 +274,6 @@ for cgroups, with a minimum of additional kernel code.
 1.4 What does notify_on_release do ?
 ------------------------------------
 
-*** notify_on_release is disabled in the current patch set. It will be
-*** reactivated in a future patch in a less-intrusive manner
-
 If the notify_on_release flag is enabled (1) in a cgroup, then
 whenever the last task in the cgroup leaves (exits or attaches to
 some other cgroup) and the last child cgroup of that cgroup
@@ -360,8 +360,8 @@ Now you want to do something with this cgroup.
 
 In this directory you can find several files:
 # ls
-notify_on_release release_agent tasks
-(plus whatever files are added by the attached subsystems)
+notify_on_release releasable tasks
+(plus whatever files added by the attached subsystems)
 
 Now attach your shell to this cgroup:
 # /bin/echo $$ > tasks
@@ -404,19 +404,13 @@ with a subsystem id which will be assigned by the cgroup system.
 Other fields in the cgroup_subsys object include:
 
 - subsys_id: a unique array index for the subsystem, indicating which
-  entry in cgroup->subsys[] this subsystem should be
-  managing. Initialized by cgroup_register_subsys(); prior to this
-  it should be initialized to -1
+  entry in cgroup->subsys[] this subsystem should be managing.
 
-- hierarchy: an index indicating which hierarchy, if any, this
-  subsystem is currently attached to. If this is -1, then the
-  subsystem is not attached to any hierarchy, and all tasks should be
-  considered to be members of the subsystem's top_cgroup. It should
-  be initialized to -1.
+- name: should be initialized to a unique subsystem name. Should be
+  no longer than MAX_CGROUP_TYPE_NAMELEN.
 
-- name: should be initialized to a unique subsystem name prior to
-  calling cgroup_register_subsystem. Should be no longer than
-  MAX_CGROUP_TYPE_NAMELEN
+- early_init: indicate if the subsystem needs early initialization
+  at system boot.
 
 Each cgroup object created by the system has an array of pointers,
 indexed by subsystem id; this pointer is entirely managed by the
@@ -434,8 +428,6 @@ situation.
 See kernel/cgroup.c for more details.
 
 Subsystems can take/release the cgroup_mutex via the functions
-cgroup_lock()/cgroup_unlock(), and can
-take/release the callback_mutex via the functions
 cgroup_lock()/cgroup_unlock().
 
 Accessing a task's cgroup pointer may be done in the following ways:
@@ -444,7 +436,7 @@ Accessing a task's cgroup pointer may be done in the following ways:
 - inside an rcu_read_lock() section via rcu_dereference()
 
 3.3 Subsystem API
---------------------------
+-----------------
 
 Each subsystem should:
 
@@ -455,7 +447,8 @@ Each subsystem may export the following methods. The only mandatory
 methods are create/destroy. Any others that are null are presumed to
 be successful no-ops.
 
-struct cgroup_subsys_state *create(struct cgroup *cont)
+struct cgroup_subsys_state *create(struct cgroup_subsys *ss,
+				   struct cgroup *cgrp)
 (cgroup_mutex held by caller)
 
 Called to create a subsystem state object for a cgroup. The
@@ -470,7 +463,7 @@ identified by the passed cgroup object having a NULL parent (since
 it's the root of the hierarchy) and may be an appropriate place for
 initialization code.
 
-void destroy(struct cgroup *cont)
+void destroy(struct cgroup_subsys *ss, struct cgroup *cgrp)
 (cgroup_mutex held by caller)
 
 The cgroup system is about to destroy the passed cgroup; the subsystem
@@ -481,7 +474,14 @@ cgroup->parent is still valid. (Note - can also be called for a
 newly-created cgroup if an error occurs after this subsystem's
 create() method has been called for the new cgroup).
 
-int can_attach(struct cgroup_subsys *ss, struct cgroup *cont,
+void pre_destroy(struct cgroup_subsys *ss, struct cgroup *cgrp);
+(cgroup_mutex held by caller)
+
+Called before checking the reference count on each subsystem. This may
+be useful for subsystems which have some extra references even if
+there are not tasks in the cgroup.
+
+int can_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
 	       struct task_struct *task)
 (cgroup_mutex held by caller)
 
@@ -492,8 +492,8 @@ unspecified task can be moved into the cgroup. Note that this isn't
 called on a fork. If this method returns 0 (success) then this should
 remain valid while the caller holds cgroup_mutex.
 
-void attach(struct cgroup_subsys *ss, struct cgroup *cont,
-	    struct cgroup *old_cont, struct task_struct *task)
+void attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
+	    struct cgroup *old_cgrp, struct task_struct *task)
 
 Called after the task has been attached to the cgroup, to allow any
 post-attachment activity that requires memory allocations or blocking.
@@ -505,9 +505,9 @@ registration for all existing tasks.
 
 void exit(struct cgroup_subsys *ss, struct task_struct *task)
 
-Called during task exit
+Called during task exit.
 
-int populate(struct cgroup_subsys *ss, struct cgroup *cont)
+int populate(struct cgroup_subsys *ss, struct cgroup *cgrp)
 
 Called after creation of a cgroup to allow a subsystem to populate
 the cgroup directory with file entries.  The subsystem should make
@@ -516,7 +516,7 @@ include/linux/cgroup.h for details).  Note that although this
 method can return an error code, the error code is currently not
 always handled well.
 
-void post_clone(struct cgroup_subsys *ss, struct cgroup *cont)
+void post_clone(struct cgroup_subsys *ss, struct cgroup *cgrp)
 
 Called at the end of cgroup_clone() to do any paramater
 initialization which might be required before a task could attach.  For