[PATCH 01/10] cgroup: add cgroup_root_mutex

[KAMEZAWA Hiroyuki]

On Tue,  1 Nov 2011 16:46:24 -0700
Tejun Heo <tj at kernel.org> wrote:

> cgroup wants to make threadgroup stable while modifying cgroup
> hierarchies which will introduce locking dependency on
> cred_guard_mutex from cgroup_mutex.  This unfortunately completes
> circular dependency.
> 
>  A. cgroup_mutex -> cred_guard_mutex -> s_type->i_mutex_key -> namespace_sem
>  B. namespace_sem -> cgroup_mutex
> 
> B is from cgroup_show_options() and this patch breaks it by
> introducing another mutex cgroup_root_mutex which nests inside
> cgroup_mutex and protects cgroupfs_root.
> 
> Signed-off-by: Tejun Heo <tj at kernel.org>
> Cc: Oleg Nesterov <oleg at redhat.com>

Thanks,
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>