linux-next: Unable to mount a cgroup file system

[Andrei Vagin]

Hello,

We run CRIU tests on linux-next kernels. Today, I found that our test
robot hangs up on mounting a cgroup file system.

https://travis-ci.org/avagin/linux/jobs/455732006

  632 ?        Ssl    0:00 /usr/bin/containerd
  843 ?        Sl     0:00  \_ containerd-shim -namespace moby
-workdir /var/lib/containerd/io.containerd.runtime.v1.linux/moby/c2311352d53eed1f5094580102e41c2a02eaf98b8626c86ccf314599101b26
  862 pts/0    Ss+    0:00      \_ python test/zdtm.py run -T
zdtm/static/cgroup.*
 1652 pts/0    S+     0:00          \_ flock zdtm_mount_cgroups.lock
./zdtm_umount_cgroups
 1653 pts/0    S+     0:00              \_ /bin/sh ./zdtm_umount_cgroups
 1659 pts/0    D+     0:06                  \_ mount -t cgroup -o
none,name=zdtmtst.defaultroot zdtm zdtm.9QFGko

[root@fc24 ~]# cat /proc/1659/stack
[<0>] msleep+0x38/0x40
[<0>] cgroup1_get_tree+0x4e1/0x749
[<0>] vfs_get_tree+0x5e/0x140
[<0>] do_mount+0x326/0xc70
[<0>] ksys_mount+0xba/0xd0
[<0>] __x64_sys_mount+0x21/0x30
[<0>] do_syscall_64+0x60/0x210
[<0>] entry_SYSCALL_64_after_hwframe+0x49/0xbe
[<0>] 0xffffffffffffffff

[root@fc24 ~]# cat /proc/1659/cgroup  | grep zdtm
13:name=zdtmtst.defaultroot:/
12:name=zdtmtst:/

[root@fc24 ~]# strace -fp 1659
strace: Process 1659 attached
mount("zdtm", "/criu/test/zdtm.9QFGko", "cgroup", MS_MGC_VAL,
"none,name=zdtmtst.defaultroot") = ? ERESTARTNOINTR (To be restarted)
mount("zdtm", "/criu/test/zdtm.9QFGko", "cgroup", MS_MGC_VAL,
"none,name=zdtmtst.defaultroot") = ? ERESTARTNOINTR (To be restarted)
mount("zdtm", "/criu/test/zdtm.9QFGko", "cgroup", MS_MGC_VAL,
"none,name=zdtmtst.defaultroot") = ? ERESTARTNOINTR (To be restarted)
mount("zdtm", "/criu/test/zdtm.9QFGko", "cgroup", MS_MGC_VAL,
"none,name=zdtmtst.defaultroot") = ? ERESTARTNOINTR (To be restarted)
mount("zdtm", "/criu/test/zdtm.9QFGko", "cgroup", MS_MGC_VAL,
"none,name=zdtmtst.defaultroot") = ? ERESTARTNOINTR (To be restarted)

Steps to reproduce:
I don't know how to reproduce this issue without running criu tests. I
tried to create a simple reproducer, but I failed. So I created a
docker container and the problem can be reproduced by running this
command:
docker run --rm -it --privileged -v /lib/modules:/lib/modules --tmpfs
/run docker.io/avagin/criu-fc29-cgroup python test/zdtm.py run -T
'zdtm/static/cgroup.*'

I found that something wrong is in
[16ec1a5d58ea67ba737d3a66efe9e53c6bb149f7] kernfs, sysfs, cgroup,
intel_rdt: Support fs_context

https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?h=next-20181116&id=16ec1a5d58ea67ba737d3a66efe9e53c6bb149f7

Thanks,
Andrei