linux-kernel - Re: [PATCH 4/6] cgroups: forbid pre

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Message-ID: <20121019202405.GR13370@google.com>
Date:	Fri, 19 Oct 2012 13:24:05 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	linux-mm@...ck.org, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org, Li Zefan <lizefan@...wei.com>,
	Johannes Weiner <hannes@...xchg.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Balbir Singh <bsingharora@...il.com>
Subject: Re: [PATCH 4/6] cgroups: forbid pre_destroy callback to fail

Hello, Michal.

On Fri, Oct 19, 2012 at 03:32:45PM +0200, Michal Hocko wrote:
> On Thu 18-10-12 15:41:48, Tejun Heo wrote:
> > Hello, Michal.
> > 
> > On Wed, Oct 17, 2012 at 03:30:46PM +0200, Michal Hocko wrote:
> > > Now that mem_cgroup_pre_destroy callback doesn't fail finally we can
> > > safely move on and forbit all the callbacks to fail. The last missing
> > > piece is moving cgroup_call_pre_destroy after cgroup_clear_css_refs so
> > > that css_tryget fails so no new charges for the memcg can happen.
> > > The callbacks are also called from within cgroup_lock to guarantee that
> > > no new tasks show up. We could theoretically call them outside of the
> > > lock but then we have to move after CGRP_REMOVED flag is set.
> > > 
> > > Signed-off-by: Michal Hocko <mhocko@...e.cz>
> > 
> > So, the plan is to do something like the following once memcg is
> > ready.
> > 
> >   http://thread.gmane.org/gmane.linux.kernel.containers/22559/focus=75251
> > 
> > Note that the patch is broken in a couple places but it does show the
> > general direction.  I'd prefer if patch #3 simply makes pre_destroy()
> > return 0 and drop __DEPRECATED_clear_css_refs from mem_cgroup_subsys.
> 
> We can still fail inn #3 without this patch becasuse there are is no
> guarantee that a new task is attached to the group. And I wanted to keep
> memcg and generic cgroup parts separated.

Yes, but all other controllers are broken that way too and the worst
thing which will hapen is triggering WARN_ON_ONCE().  Let's note the
failure in the commit and remove DEPREDATED_clear_css_refs in the
previous patch.  Then, I can pull from you, clean up pre_destroy mess
and then you can pull back for further cleanups.

Thanks.

-- 
tejun
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