Date: Wed, 31 Oct 2012 21:08:59 +0100
From: Michal Hocko <mhocko@suse.cz>
To: Tejun Heo <tj@kernel.org>
Cc: lizefan@huawei.com, hannes@cmpxchg.org, bsingharora@gmail.com,
        kamezawa.hiroyu@jp.fujitsu.com, containers@lists.linux-foundation.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/8] cgroup: kill
 cgroup_subsys->__DEPRECATED_clear_css_refs
Message-ID: <20121031200859.GE1271@dhcp22.suse.cz>
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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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