From ca8bdcaff0d77990fb69e0f946018c96a70851cc Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Mon, 26 Aug 2013 18:40:56 -0400
Subject: [PATCH] cgroup: make cgroup_css() take cgroup_subsys * instead and
 allow NULL subsys

cgroup_css() is no longer used in hot paths.  Make it take struct
cgroup_subsys * and allow the users to specify NULL subsys to obtain
the dummy_css.  This removes open-coded NULL subsystem testing in a
couple users and generally simplifies the code.

After this patch, css_from_dir() also allows NULL @ss and returns the
matching dummy_css.  This behavior change doesn't affect its only user
- perf.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
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