How to unmount a recursive bind mount - safely

Question

Please take care that you have no unsaved work if you test this.

I ran this command as an experiment, but how can I undo it without rebooting?

# mount --rbind / /mnt

What I have tried

1. # umount /mnt
   umount: /mnt: target is busy.
   

2. # strace -f -e umount,umount2 umount -R /mnt
   umount2("/mnt/var/lib/nfs/rpc_pipefs", 0) = 0
   umount2("/mnt/sys/kernel/security", 0)  = 0
   umount2("/mnt/sys/fs/cgroup/unified", 0) = -1 EBUSY (Device or resource busy)
   umount: /mnt/sys/fs/cgroup/unified: target is busy.
   +++ exited with 32 +++
   

3. # umount -l /mnt
   # findmnt
   findmnt: can't read /proc/mounts: No such file or directory
   

   (this is the catastrophe I warned you about).

Environment

Fedora 28.

Answer 1

You need to run mount --make-rprivate /mnt before you can safely unmount it. See the question Mounting new filesystem affects non-recursive bind mounts?

I thought I had tried this, but something went wrong. Maybe I had missed the r off --make-rprivate.