linux-kernel - Re: [PATCH 2/8] cgroup: kill CSS

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Date:	Mon, 5 Nov 2012 13:33:44 +0800
From:	Li Zefan <lizefan@...wei.com>
To:	Tejun Heo <tj@...nel.org>
CC:	<hannes@...xchg.org>, <mhocko@...e.cz>, <bsingharora@...il.com>,
	<kamezawa.hiroyu@...fujitsu.com>,
	<containers@...ts.linux-foundation.org>, <cgroups@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/8] cgroup: kill CSS_REMOVED

On 2012/11/1 3:44, Tejun Heo wrote:
> CSS_REMOVED is one of the several contortions which were necessary to
> support css reference draining on cgroup removal.  All css->refcnts
> which need draining should be deactivated and verified to equal zero
> atomically w.r.t. css_tryget().  If any one isn't zero, all refcnts
> needed to be re-activated and css_tryget() shouldn't fail in the
> process.
> 
> This was achieved by letting css_tryget() busy-loop until either the
> refcnt is reactivated (failed removal attempt) or CSS_REMOVED is set
> (committing to removal).
> 
> Now that css refcnt draining is no longer used, there's no need for
> atomic rollback mechanism.  css_tryget() simply can look at the
> reference count and fail if it's deactivated - it's never getting
> re-activated.
> 
> This patch removes CSS_REMOVED and updates __css_tryget() to fail if
> the refcnt is deactivated.  As deactivation and removal are a single
> step now, they no longer need to be protected against css_tryget()
> happening from irq context.  Remove local_irq_disable/enable() from
> cgroup_rmdir().
> 
> Note that this removes css_is_removed() whose only user is VM_BUG_ON()
> in memcontrol.c.  We can replace it with a check on the refcnt but
> given that the only use case is a debug assert, I think it's better to
> simply unexport it.
> 
> v2: Comment updated and explanation on local_irq_disable/enable()
>     added per Michal Hocko.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Reviewed-by: Michal Hocko <mhocko@...e.cz>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Balbir Singh <bsingharora@...il.com>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>

Acked-by: Li Zefan <lizefan@...wei.com>

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