linux-kernel - Re: [PATCHSET cgroup/for-3.16] cgroup: remove cgroup_tree

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Message-ID: <20140513162219.GB1866@htj.dyndns.org>
Date:	Tue, 13 May 2014 12:22:19 -0400
From:	Tejun Heo <tj@...nel.org>
To:	lizefan@...wei.com
Cc:	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHSET cgroup/for-3.16] cgroup: remove cgroup_tree_mutex

On Tue, May 06, 2014 at 04:19:26PM -0400, Tejun Heo wrote:
> Hello,
> 
> cgroup_tree_mutex was introduced during kernfs conversion to work
> around the cyclic locking dependency between kernfs active protection
> and cgroup_mutex.  Some file and directory operations need to acquire
> cgroup_mutex which puts the mutex under the kernfs active protection;
> however, cgroup also needs to access the hierarchy and the registered
> cftypes to detemine which files to remove, which obviously can't be
> done while holding cgroup_mutex anymore.
> 
> cgroup_tree_mutex nests above both cgroup_mutex and kernfs active
> protection and protects hierarchy and cftypes so that those file
> operations can be performed while holding it without cgroup_mutex.
> This works but is kinda cumbersome as most places end up taking both
> cgroup_tree_mutex and cgroup_mutex and there's on-going friction on
> what needs to be protected by which combination.
> 
> Furthermore, due to new requirements from subtree_control
> implementations, kernfs ended up growing full-blown mechanism to
> bypass active protection instead of just supporting self-removal and
> cgroup ended up using both mechanisms - two layered mutexes and active
> protection bypss - on different areas, which is totally unncessary.
> 
> This patchset converts everything over to kernfs active protection
> bypass and drops cgroup_tree_mutex making cgroup locking noticeably
> simpler.  It contains the following eight patches.
> 
>  0001-cgroup-reorganize-cgroup_create.patch
>  0002-cgroup-collapse-cgroup_create-into-croup_mkdir.patch
>  0003-cgroup-grab-cgroup_mutex-earlier-in-cgroup_subtree_c.patch
>  0004-cgroup-move-cgroup-kn-priv-clearing-to-cgroup_rmdir.patch
>  0005-cgroup-factor-out-cgroup_kn_lock_live-and-cgroup_kn_.patch
>  0006-cgroup-use-cgroup_kn_lock_live-in-other-cgroup-kernf.patch
>  0007-cgroup-nest-kernfs-active-protection-under-cgroup_mu.patch
>  0008-cgroup-remove-cgroup_tree_mutex.patch

Applied to cgroup/for-3.16.

Thanks.

-- 
tejun
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