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 the init process hasn't 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>
---
 kernel/cgroup.c |   30 ++++--------------------------
 1 files changed, 4 insertions(+), 26 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 78e5bde..2b72346 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2467,7 +2467,6 @@ 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);
 
@@ -2478,32 +2477,11 @@ 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;
 
-- 
1.5.4.rc3
_______________________________________________
Containers mailing list
Containers@lists.linux-foundation.org
https://lists.linux-foundation.org/mailman/listinfo/containers

On Tue, Apr 1, 2008 at 7:16 PM, Li Zefan <lizf@cn.fujitsu.com> wrote:
>  -
>  -       /* 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);
>  -       }

Should we maybe call ss->fork(ss, &init_task) ? Or just document that
ss->fork() explicitly doesn't get called for init.

Maybe also add some BUG()s to confirm that no other tasks have in fact
been forked by this point?

Paul
_______________________________________________
Containers mailing list
Containers@lists.linux-foundation.org
https://lists.linux-foundation.org/mailman/listinfo/containers

Paul Menage wrote:
> On Tue, Apr 1, 2008 at 7:16 PM, Li Zefan <lizf@cn.fujitsu.com> wrote:
>>  -
>>  -       /* 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);
>>  -       }
> 
> Should we maybe call ss->fork(ss, &init_task) ? Or just document that
> ss->fork() explicitly doesn't get called for init.
> 
> Maybe also add some BUG()s to confirm that no other tasks have in fact
> been forked by this point?
> 

At system boot, before all the subsystems have been registered, no processes
including init has been forked, am I right? So the fork callback will be
invoked when the init process is forked, so we don't need to call
ss->fork(ss, &init_task).

I'll add a BUG_ON(), and update the document about the fork callback.

Regards,
Li Zefan
_______________________________________________
Containers mailing list
Containers@lists.linux-foundation.org
https://lists.linux-foundation.org/mailman/listinfo/containers