linux-kernel - Re: [PATCH] hugetlb/cgroup: Simplify pre

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Date:	Thu, 19 Jul 2012 13:42:28 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Li Zefan <lizefan@...wei.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hugetlb/cgroup: Simplify pre_destroy callback

On Thu 19-07-12 16:56:18, Aneesh Kumar K.V wrote:
> Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com> writes:
> 
> >>>>>
> >>>>> We test RES_USAGE before taking hugetlb_lock.  What prevents some other
> >>>>> thread from increasing RES_USAGE after that test?
> >>>>>
> >>>>> After walking the list we test RES_USAGE after dropping hugetlb_lock.
> >>>>> What prevents another thread from incrementing RES_USAGE before that
> >>>>> test, triggering the BUG?
> >>>>
> >>>> IIUC core cgroup will prevent a new task getting added to the cgroup
> >>>> when we are in pre_destroy. Since we already check that the cgroup doesn't
> >>>> have any task, the RES_USAGE cannot increase in pre_destroy.
> >>>>
> >>>
> >>>
> >>> You're wrong here. We release cgroup_lock before calling pre_destroy and retrieve
> >>> the lock after that, so a task can be attached to the cgroup in this interval.
> >>>
> >>
> >> But that means rmdir can be racy right ? What happens if the task got
> >> added, allocated few pages and then moved out ? We still would have task
> >> count 0 but few pages, which we missed to to move to parent cgroup.
> >>
> >
> > That's a problem even if it's verrrry unlikely.
> > I'd like to look into it and fix the race in cgroup layer.
> > But I'm sorry I'm a bit busy in these days...
> >
> 
> How about moving that mutex_unlock(&cgroup_mutex) to memcg callback ? That
> can be a patch for 3.5 ? 

Bahh, I have just posted a follow up on mm-commits email exactly about
this. Sorry I have missed that the discussion is still ongoing. I have
posted also something I guess should help. Can we follow up on that one
or should I post the patch here as well?

> 
> -aneesh
>  
> 

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic
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