[PATCH cgroup/for-3.12 1/2] cgroup: fix subsystem file accesses on the root cgroup

[Li Zefan]

> 105347ba5 ("cgroup: make cgroup_file_open() rcu_read_lock() around
> cgroup_css() and add cfent->css") added cfent->css to cache the
> associted cgroup_subsys_state across file operations.
> 
> A cfent is associated with single css throughout its lifetime and the
> origimal commit initialized the cache pointer during cgroup_add_file()
> and verified that it matches the actual one in cgroup_file_open().
> While this works fine for !root cgroups, it's broken for root cgroups
> as files in a root cgroup are created before the css's are associated
> with the cgroup and thus cgroup_css() call in cgroup_add_file()
> returns NULL associating all cfents in the root cgroup with NULL css.
> This makes cgroup_file_open() trigger WARN and fail with -ENODEV for
> all !core subsystem files in the root cgroups.
> 
> There's no reason to initialize cfent->css separately from
> cgroup_add_file().  As the association never changes,
> cgroup_file_open() can set it unconditionally every time and
> containing the logic in cgroup_file_open() makes more sense anyway as
> the only reason it's necessary is file->private_data being already
> occupied.
> 
> Fix it by setting cfent->css unconditionally from cgroup_file_open().
> 
> Signed-off-by: Tejun Heo <tj at kernel.org>

Acked-by: Li Zefan <lizefan at huawei.com>