netdev - Re: [PATCH v1 3/5] cgroup: Protect access to task_cls

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Date:	Tue, 21 Aug 2012 13:12:33 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Neil Horman <nhorman@...driver.com>
CC:	Daniel Wagner <wagi@...om.org>, <netdev@...r.kernel.org>,
	<cgroups@...r.kernel.org>,
	Daniel Wagner <daniel.wagner@...-carit.de>,
	"David S. Miller" <davem@...emloft.net>,
	Gao feng <gaofeng@...fujitsu.com>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	John Fastabend <john.r.fastabend@...el.com>,
	Li Zefan <lizefan@...wei.com>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v1 3/5] cgroup: Protect access to task_cls_classid() when
 built as module

On 08/20/2012 09:03 PM, Neil Horman wrote:
>>> 2) Keep a separate internal counter to track the number of cgroup instances
>>> > > so that you only inc the static key on the first create and dec it on the last
>>> > > delete.
>> > 
>> > If I got you right, than this would not be different then direclty using
>> > static_key_slow_[inc|dec].
>> > 
> As long as a cgroup subsystems ->destroy method is only called when the
> subsystem is being removed, then I think thats correct.  I'm not 100% sure thats
> the case though.
> 
THAT is correct, but not the call itself. ->destroy() is called with the
cgroup_lock held, and there is a lockdep dependency created by cpuset
that prevents the cpu_hotplug lock, taken by static branch updates, to
be taken inside cgroup_lock.

So unless cpuset is fixed - which is a major work, we can't do
static_branch updates while holding the cgroup_lock.


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