linux-kernel - Re: bind() call in cgroup's css structure

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Message-id: <4F8385F5.1020901@huawei.com>
Date:	Tue, 10 Apr 2012 08:59:33 +0800
From:	Li Zefan <lizefan@...wei.com>
To:	Glauber Costa <glommer@...allels.com>
Cc:	Tejun Heo <tj@...nel.org>, Paul Turner <pjt@...gle.com>,
	cgroups@...r.kernel.org, "devel@...nvz.org" <devel@...nvz.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: bind() call in cgroup's css structure

Glauber Costa wrote:

> Hello Tejun,
> 
> During your cgroup refactor, I was wondering if you have any plans to get rid of the bind() callback that is called when hierarchies are moved?


I planned to remove it long ago, and Paul M agreed. But after some time, I was
trying to make use of it in a patchset, which was used to fix the problem that
remount with different subsys bits will fail for !root cgroups.

> 
> At least in tree, there seems to be no users for that.
> I actually planned to use it myself, to start or remove a jump label
> when cpuacct and cpu cgroups were comounted.
> 
> Problem is, because we have some calls in the cpuset cgroup from inside the cpu hotplug handler, we end up taking the almighty cgroup_mutex from inside the cpu_hotplug.lock.
> 
> jump labels take it in most arches through the get_online_cpus() function call. This means we effectively can't apply jump labels with the cgroup_mutex held, which is the case throughout the whole bind() call.
> 
> All that explained, I figured I might as well ask before I attempted a solution to that myself: as much as populate(), bind seems to be one of the overly complicated callbacks, designed for a scenario in which everything can come and go at will, which is something we're trying to fix.
> 


As we aim for single hierarchy, it defenitely should be removed.

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