Comment # 8 on bug 1201114 from Stanislav Brabec

Thanks. Reassigning.

Just guessing that the problem is somewhere in the kernel, in the way, how it
holds information about rslave. It seems to stay connected to the device, even
if it is successfully remounted. It could easily happen, because technically,
setting of propagation flag (--make-rslave) uses a special form of mount()
syscall:

mount("none", "/mnt/chroot/sys", NULL, MS_REC|MS_SLAVE, NULL) = 0

I do not see any reverse command that could be called before the unplug.