failed build:
https://cros-goldeneye.corp.google.com/chromeos/healthmonitoring/buildDetails?buildbucketId=8932414403546587488

from CleanUp log (https://luci-logdog.appspot.com/v/?s=chromeos/bb/chromeos/caroline-arcnext-paladin/1591/+/recipes/steps/CleanUp/0/stdout):

umount: /b/c/cbuild/repository/chroot: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))
cros_sdk: Unhandled exception:
Traceback (most recent call last):
  File "/b/c/cbuild/repository/chromite/bin/cros_sdk", line 169, in <module>
    DoMain()
  File "/b/c/cbuild/repository/chromite/bin/cros_sdk", line 165, in DoMain
    commandline.ScriptWrapperMain(FindTarget)
  File "/b/c/cbuild/repository/chromite/lib/commandline.py", line 912, in ScriptWrapperMain
    ret = target(argv[1:])
  File "/b/c/cbuild/repository/chromite/scripts/cros_sdk.py", line 976, in main
    osutils.UmountTree(options.chroot)
  File "/b/c/cbuild/repository/chromite/lib/osutils.py", line 902, in UmountTree
    UmountDir(mount_pt, lazy=False, cleanup=False)
  File "/b/c/cbuild/repository/chromite/lib/osutils.py", line 864, in UmountDir
    runcmd(cmd, print_cmd=False)
  File "/b/c/cbuild/repository/chromite/lib/cros_build_lib.py", line 283, in SudoRunCommand
    return RunCommand(cmd, **kwargs)
  File "/b/c/cbuild/repository/chromite/lib/cros_build_lib.py", line 647, in RunCommand
    raise RunCommandError(msg, cmd_result)
chromite.lib.cros_build_lib.RunCommandError: return code: 1; command: umount -d /b/c/cbuild/repository/chroot
cmd=['umount', '-d', '/b/c/cbuild/repository/chroot']
06:37:31: INFO: Cleaning up buildroot git repositories.
06:37:31: INFO: Resetting all repo branches: /b/c/cbuild/repository
06:37:31: INFO: Wiping old output.
06:37:31: INFO: Deleting archived trybot images.
06:37:31: INFO: Deleting archived perf results.
06:37:31: INFO: Deleting autotest site packages.
06:37:31: INFO: Deleting chroot.
umount: /b/c/cbuild/repository/chroot: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))
06:37:39: ERROR: <class 'chromite.lib.cros_build_lib.RunCommandError'>: return code: 1; command: sudo -n 'CROS_CACHEDIR=/b/c/cbuild/repository/.cache' 'CROS_SUDO_KEEP_ALIVE=unknown' -- umount -d /b/c/cbuild/repository/chroot