Comment 1 for bug 1185394

Debugging ideas (from discussion with pitti on IRC):

(1) add a ps | grep ... wait loop into the hook until really all udev processes are gone that would confirm that the child processes actually run into the /dev issue
(2) some ulimit -c unlimited and copying core dumps to /run/udev/
(3) move 60-persistent-storage-dm.rules from IMPORT{program}="/sbin/blkid -o ... to IMPORT{builtin}="blkid --offset..., to see whether it's the external program which causes trouble
and (4) it would be helpful if you could confirm that you have udev processes running after the udevadm control --exit (with http://paste.ubuntu.com/5713055/)