This partly broke the last CQ run:
https://cros-goldeneye.corp.google.com/chromeos/healthmonitoring/buildDetails?buildbucketId=8923991602867193632

Error:

06:26:11: INFO: Cleaning up chroot at /b/swarming/w/ir/cache/cbuild/repository/chroot
umount: /b/swarming/w/ir/cache/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))

06:26:11: INFO: Waiting for ts_mon flushing process to finish...
06:26:53: INFO: Finished waiting for ts_mon process.
cbuildbot_launch: Unhandled exception:
Traceback (most recent call last):
  File "/b/swarming/w/ir/kitchen-workdir/chromite/scripts/cbuildbot_launch", line 170, in <module>
    DoMain()
  File "/b/swarming/w/ir/kitchen-workdir/chromite/scripts/cbuildbot_launch", line 166, in DoMain
    commandline.ScriptWrapperMain(FindTarget)
  File "/b/swarming/w/ir/kitchen-workdir/chromite/lib/commandline.py", line 912, in ScriptWrapperMain
    ret = target(argv[1:])
  File "/b/swarming/w/ir/kitchen-workdir/chromite/scripts/cbuildbot_launch.py", line 498, in main
    return _main(options, argv)
  File "/b/swarming/w/ir/kitchen-workdir/chromite/scripts/cbuildbot_launch.py", line 481, in _main
    CleanupChroot(buildroot)
  File "/b/swarming/w/ir/kitchen-workdir/chromite/scripts/cbuildbot_launch.py", line 72, in wrapped_functor
    result = functor(*args, **kwargs)
  File "/b/swarming/w/ir/kitchen-workdir/chromite/scripts/cbuildbot_launch.py", line 399, in CleanupChroot
    cros_sdk_lib.CleanupChrootMount(chroot_dir, delete=False)
  File "/b/swarming/w/ir/kitchen-workdir/chromite/lib/timeout_util.py", line 191, in TimeoutWrapper
    return func(*args, **kwargs)
  File "/b/swarming/w/ir/kitchen-workdir/chromite/lib/cros_sdk_lib.py", line 394, in CleanupChrootMount
    osutils.UmountTree(chroot)
  File "/b/swarming/w/ir/kitchen-workdir/chromite/lib/osutils.py", line 901, in UmountTree
    UmountDir(mount_pt, lazy=False, cleanup=False)
  File "/b/swarming/w/ir/kitchen-workdir/chromite/lib/osutils.py", line 863, in UmountDir
    runcmd(cmd, print_cmd=False)
  File "/b/swarming/w/ir/kitchen-workdir/chromite/lib/cros_build_lib.py", line 321, in SudoRunCommand
    return RunCommand(sudo_cmd, **kwargs)
  File "/b/swarming/w/ir/kitchen-workdir/chromite/lib/cros_build_lib.py", line 646, in RunCommand
    raise RunCommandError(msg, cmd_result)
chromite.lib.cros_build_lib.RunCommandError: return code: 1; command: sudo -- umount -d /b/swarming/w/ir/cache/cbuild/repository/chroot
cmd=['sudo', '--', 'umount', '-d', '/b/swarming/w/ir/cache/cbuild/repository/chroot']