linux-kernel - Re: [PATCH 3/8] cgroup: use cgroup_lock_live_group(parent) in cgroup

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Message-ID: <20121101091644.GA8533@dhcp22.suse.cz>
Date:	Thu, 1 Nov 2012 10:16:44 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	lizefan@...wei.com, hannes@...xchg.org, bsingharora@...il.com,
	kamezawa.hiroyu@...fujitsu.com,
	containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/8] cgroup: use cgroup_lock_live_group(parent) in
 cgroup_create()

On Wed 31-10-12 10:04:31, Tejun Heo wrote:
> Hey, Michal.
> 
> On Wed, Oct 31, 2012 at 04:55:14PM +0100, Michal Hocko wrote:
> > > +	/*
> > > +	 * Only live parents can have children.  Note that the liveliness
> > > +	 * check isn't strictly necessary because cgroup_mkdir() and
> > > +	 * cgroup_rmdir() are fully synchronized by i_mutex; however, do it
> > > +	 * anyway so that locking is contained inside cgroup proper and we
> > > +	 * don't get nasty surprises if we ever grow another caller.
> > > +	 */
> > > +	if (!cgroup_lock_live_group(parent)) {
> > > +		err = -ENODEV;
> > > +		goto err_free;
> > > +	}
> > > +
> > 
> > I think this should be moved up before we try to allocate any memory.
> > Or is your motivation to keep cgroup_lock held for shorter time?
> > I could agree with that but a small comment would be helpful.
> 
> Then I have to change the error out path more and I'm not sure I wanna
> call deactivate_super() under cgroup_mutex.  It's just simpler this
> way.

I am not sure I understand. What does deactivate_super has to do with
the above suggestion? cgroup_lock_live_group will take the cgroup_mutex
on the success or frees the previously allocated&unused memory. The
only thing I was suggesting is to do cgroup_lock_live_group first and
allocate the cgroup only if it doesn't fail.

-- 
Michal Hocko
SUSE Labs
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