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

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Date:	Wed, 31 Oct 2012 21:08:59 +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 1/8] cgroup: kill
 cgroup_subsys->__DEPRECATED_clear_css_refs

On Wed 31-10-12 12:44:03, Tejun Heo wrote:
[...]
> local_irq_save/restore() from cgroup_clear_css_refs() are replaced
> with local_irq_disable/enable() for simplicity.  This is safe as
> cgroup_rmdir() is always called with IRQ enabled.  Note that this IRQ
> switching is necessary to make CSS deactivation and CSS_REMOVED
> assertion atomic against css_tryget() and will be removed by future
> changes.

local_irq_disable doesn't guarantee atomicity, because other CPUs will
see the change in steps so this is a bit misleading. The real reason
AFAICS is that we do not want to hang in css_tryget from IRQ context
(does this really happen btw.?) which would interrupt cgroup_rmdir
between we add CSS_DEACT_BIAS and the group is marked CGRP_REMOVED.
Or am I still missing the point?
[...]
-- 
Michal Hocko
SUSE Labs
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