linux-kernel - Re: A "domain invalid" cgroup *can* sometimes have member tasks

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Message-ID: <20180220193547.GB170644@devbig577.frc2.facebook.com>
Date:   Tue, 20 Feb 2018 11:35:47 -0800
From:   Tejun Heo <tj@...nel.org>
To:     "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc:     "Serge E. Hallyn" <serge@...lyn.com>,
        lkml <linux-kernel@...r.kernel.org>,
        "open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>
Subject: Re: A "domain invalid" cgroup *can* sometimes have member tasks

On Tue, Feb 20, 2018 at 08:26:59PM +0100, Michael Kerrisk (man-pages) wrote:
> Hello Tejun
> 
> According to Documentation/cgroup-v2.txt, a "domain invalid" cgroup
> can't have member tasks. And indeed this is generally not permitted.
> 
> However, someone recently showed me a scenario where a "domain
> invalid" cgroup can have member processes. See the following example:
> 
> # mkdir -p /sys/fs/cgroup/unified/grp0/grp1
> # sleep 1000 &
> [1] 10549
> # echo 10549 > /sys/fs/cgroup/unified/grp0/grp1/cgroup.procs
> # echo threaded > /sys/fs/cgroup/unified/grp0/cgroup.type
> # cat /sys/fs/cgroup/unified/grp0/cgroup.type
> threaded
> # cat /sys/fs/cgroup/unified/grp0/grp1/cgroup.type
> domain invalid
> # cat /sys/fs/cgroup/unified/grp0/grp1/cgroup.threads
> 10549
> 
> From the above, we see that the cgroup grp0/grp1 is of type "domain
> invalid" and has a member thread. This seems to be a violation of the
> documented rules, and is I assume a bug, since in the above scenario,
> we are denied from adding further tasks to the grp0/grp1 cgroup:
> 
> # sleep 2000 &
> [2] 10553
> # echo 10553 > /sys/fs/cgroup/unified/grp0/grp1/cgroup.procs
> sh: echo: write error: Operation not supported
> 
> Could you comment please?

Hmm... nr_populated_domain_children check should have caught that
condition and rejected it.  Will look into what's going on.

Thanks.

-- 
tejun