Re: [PATCH] cgroup: protect modifications to cgroup_idr with cgroup_mutex

[Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>]

On 2014/2/11 18:20, Michal Hocko wrote:
> Hi Li,
> good work in reproducing the issue so quickly!
> I have tried to backport this patch to 3.12 kernel but the code has
> changed since then.
> The only two instances of idr_remove which are called outside of
> cgroup_mutex seem to be:
>  - cgroup_create calling it from err_free_id: path
>  - css_free_work_fn
>      mem_cgroup_css_free
>        __mem_cgroup_free
>          free_css_id
> 
> The second one takes ss->id_lock spinlock which should be sufficient
> to exclude get_new_cssid but cgroup_mount and cgroup_create don't use
> id_lock. They do hold cgroup_mutex though. So I guess I need something
> like the following (I will have it tested):

I don't think you need to do anything with ss->idr.

cgroup_create() calls alloc_css_id() -> get_new_cssid(), and get_new_cssid()
uses id_lock.

cgroup_mount() won't touch ss->idr, because the css_id for root cgroup is
always 0.