linux-kernel - Re: [RFC][PATCH 1/2] CGROUP ID and Hierarchy Code

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Date:	Fri, 28 Nov 2008 11:41:39 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"menage@...gle.com" <menage@...gle.com>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	taka@...inux.co.jp
Subject: Re: [RFC][PATCH 1/2] CGROUP ID and Hierarchy Code

On Fri, 28 Nov 2008 10:38:26 +0800
Li Zefan <lizf@...fujitsu.com> wrote:

> KAMEZAWA Hiroyuki wrote:
> > On Fri, 28 Nov 2008 10:35:33 +0900
> > KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> > 
> >> On Fri, 28 Nov 2008 09:28:33 +0800
> >> Li Zefan <lizf@...fujitsu.com> wrote:
> > 
> >>> I think the safe way is:
> >>>
> >>> rcu_read_lock();
> >>> cgrp = cgroup_get_next()
> >>> if (!inc_not_zero(cgrp) {
> >>> 	rcu_read_unlock();
> >>> 	return NULL;
> >>> }
> >>> rcu_read_unlock();
> >>> return cgrp;
> >>>
> >>> But it's also safe to use cgrp = list_entry(&parent->children.next) for the above
> >>> scenario, seems you don't have to invent this cgroup_get_next().
> >>>
> >> But list-walk can't provide us view of hierarchy. Up-Down list walk is better ?
> >> please see memcg's code. it met some troubles.
> >> I'll make kfree(cgrp) to be called by RCU.
> >>
> > 
> > Ah please, I have a question.
> > Following is right ?
> > 	- until mount, cgroup's subsystem root is tied to dummy? root cgroup.
> 
> Yes.
> 
> > 	- cgroup cannot be unmounted while there are any children.
> 
> No, you can unmount. You can have it a try. :)
> 
I tried and did ;)

> > 	- cgroup_root is freed/allocated at umount/mount, then I have to handle
> 
> No, kill_sb() is called when deactivate_super() decreases sb->s_active to 0, and
> whenever we create a cgroup, sb->s_active is increased, so if there are sub-dirs
> the cgroup_root won't be freed at umount.
> 
Thank you! very helpful. 

So, I should take care ofe only kill_sb() case. 
(it frees pointer of root (and root->topcgroup.) and I have to update it.)

Regards,
-Kame


> > 	  this event in cgroup-id.
> > 	 (maybe current code has leak? in umount.)
> >  
> 
> So I think there is no leak. ;)
> 
> > Thanks,
> > -Kame
> > 
> > 
> > 
> 

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