linux-kernel - Re: [PATCH 1/8] cgroup: kill cgroup_subsys->__DEPRECATED_clear_css

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Date:	Wed, 31 Oct 2012 09:41:23 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Michal Hocko <mhocko@...e.cz>
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 1/8] cgroup: kill
 cgroup_subsys->__DEPRECATED_clear_css_refs

Hey, Michal.

On Wed, Oct 31, 2012 at 03:37:51PM +0100, Michal Hocko wrote:
> > +	for_each_subsys(cgrp->root, ss)
> > +		if (ss->pre_destroy)
> > +			WARN_ON_ONCE(ss->pre_destroy(cgrp));
> 
> Hmm, I am not sure I like this WARN_ON_ONCE. First it can happen for
> more than one controller and second we can just clear CGRP_WAIT_ON_RMDIR
> and return with EBUSY. The only possible failure at the moment is when a
> new task or a child group appear.
> I know it is not a big deal because it will disappear later in the
> series but it would be more readable IMO.

The WARN_ON_ONCE() is just moved from the original
cgroup_call_pre_destroy().  We can add an error out there but that
makes future changes difficult.  It's a chicken and egg problem.  You
gotta break the loop somewhere.

Thanks.

-- 
tejun
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