Cgroup v2 thread mode oddity: "domain invalid" cgroup with threaded controller enabled

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

Hello Tejun,

Suppose we have the following scenario:

x [d]    (pids)
    y [dt]    (pids)
        p [t]
        q [t]
        r [t]
    z [d]

Here, x/y is a "domain threaded root" with a threaded controller
(the 'pids' controller) enabled. (In this scenario, there are no
member processes in any of the cgroups.)

Suppose we now convert x/z to "threaded" type:

     # echo threaded > x/z/cgroup.type

Now we end up in the following state:

x [dt]    (pids)
    y [inv]    (pids)
        p [t]
        q [t]
        r [t]
    z [t]

This seems odd. x/y is now of "domain invalid" type with a controller
enabled! This feels like a violation of the rules, since we can't
in other circumstances do anything with a "domain invalid" cgroup
except convert it to "threaded". In particular, we can't create
child cgroups under a "domain invalid" cgroup, or add member processes
to the cgroup, or *enable controllers in the cgroup*. In fact, when
doing the 

    # echo threaded > x/z/cgroup.type

I had expected a write(2) error because the state of x/y should
(I thought) not be permitted.

Your thoughts?

Thanks,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/