Cgroups v2 bug: incorrect reporting in "domain threaded" cgroup.procs file

[Michael Kerrisk (man-pages)]

Hello Tejun,

I've discovered what looks to be a bug in the reporting
of PIDs in the cgroup.procs file in the "domain threaded"
node at the root of a threaded subtree. The following
demo is on vanilla kernel 4.19.

Suppose we have the following multithreaded process:

$ ps -L 654
  PID   LWP TTY      STAT   TIME COMMAND
  654   654 pts/12   Tl     0:00 ./cpu_multithread_burner 100
  654   655 pts/12   Tl     0:01 ./cpu_multithread_burner 100

Now suppose we create a threaded subtree in the v2 hierarchy:

# cd /sys/fs/cgroup/unified/
# mkdir -p x/a/b
# echo 'threaded' > x/a/cgroup.type
# echo 'threaded' > x/a/b/cgroup.type

Then we move the multithreaded process into x/a in 
the threaded subtree:

# echo 654 > x/a/cgroup.procs 

Now we visualize the set-up using my visualization
program[1]:

# go run ~mtk/lsp/cgroups/view_v2_cgroups.go x
x [dt]
    PIDs: {654}
    a [t]
        TIDs: {654 655-[654]}
        b [t]

The above is as I expect.

Now, we move the thread group leader (it has to be the 
thread group leader to show the bug) to a x/a/b, and again
use my visualization program:

# echo 654 > x/a/b/cgroup.threads 
# go run ~mtk/lsp/cgroups/view_v2_cgroups.go x
x [dt]
    PIDs: {654 655}
    a [t]
        TIDs: {655-[654]}
        b [t]
            TIDs: {654}

Note how the *thread ID* of the non-thread-group-leader
(655) is being reported in the x/cgroup.procs!

And just to verify that this is not a bug in my 
visualization program:

# cat x/cgroup.procs 
655
654

Your thoughts?

Thanks,

Michael

[1] https://www.spinics.net/lists/cgroups/msg20710.html

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