cgroups: simplify init_subsys()

We are at system boot and there is only 1 cgroup group (i,e, init_css_set), so
we don't need to run through the css_set linked list.  Neither do we need to
run through the task list, since no processes have been created yet.

Also referring to a comment in cgroup.h:

struct css_set
{
	...
	/*
	 * Set of subsystem states, one for each subsystem. This array
	 * is immutable after creation apart from the init_css_set
	 * during subsystem registration (at boot time).
	 */
	struct cgroup_subsys_state *subsys[CGROUP_SUBSYS_COUNT];
}

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Reviewed-by: Paul Menage <menage@google.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/cgroups.txt

@@ -500,8 +500,7 @@ post-attachment activity that requires memory allocations or blocking.
 
 void fork(struct cgroup_subsy *ss, struct task_struct *task)
 
-Called when a task is forked into a cgroup. Also called during
-registration for all existing tasks.
+Called when a task is forked into a cgroup.
 
 void exit(struct cgroup_subsys *ss, struct task_struct *task)
 

kernel/cgroup.c

@@ -2477,7 +2477,6 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
 static void __init cgroup_init_subsys(struct cgroup_subsys *ss)
 {
 	struct cgroup_subsys_state *css;
-	struct list_head *l;
 
 	printk(KERN_INFO "Initializing cgroup subsys %s\n", ss->name);
 
@@ -2488,35 +2487,19 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss)
 	BUG_ON(IS_ERR(css));
 	init_cgroup_css(css, ss, dummytop);
 
-	/* Update all cgroup groups to contain a subsys
+	/* Update the init_css_set to contain a subsys
 	 * pointer to this state - since the subsystem is
-	 * newly registered, all tasks and hence all cgroup
-	 * groups are in the subsystem's top cgroup. */
-	write_lock(&css_set_lock);
-	l = &init_css_set.list;
-	do {
-		struct css_set *cg =
-			list_entry(l, struct css_set, list);
-		cg->subsys[ss->subsys_id] = dummytop->subsys[ss->subsys_id];
-		l = l->next;
-	} while (l != &init_css_set.list);
-	write_unlock(&css_set_lock);
-
- 	/* 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);
-	}
+	 * newly registered, all tasks and hence the
+	 * init_css_set is in the subsystem's top cgroup. */
+	init_css_set.subsys[ss->subsys_id] = dummytop->subsys[ss->subsys_id];
 
 	need_forkexit_callback |= ss->fork || ss->exit;
 
+	/* At system boot, before all subsystems have been
+	 * registered, no tasks have been forked, so we don't
+	 * need to invoke fork callbacks here. */
+	BUG_ON(!list_empty(&init_task.tasks));
+
 	ss->active = 1;
 }