Hello,

On Tue, Jul 10, 2012 at 10:11:05AM +0800, Li Zefan wrote:

Right, but that should be safe. The css objects of the root cgroup are
allocated at boot, and won't be destroyed at umount.

Furthermore when a cgroup hierarchy is going to be unmounted, those css's
will be made to point to a cgroup named dummytop in rebind_subsystems(),
and there's a syncronize_rcu() in the end of the function, so accessing
css->cgroup is always safe.

In this case, dummytop->dentry is NULL, and that's safe too, because
cgroup_path() is aware of this case.

I see.  Thanks for the explanation.

-- 
tejun