Comment 19 for bug 1780332

Look for "dmsetup udevcomplete" call in udev rules. This is the sync point when libdevmapper continues. This must be the last call in udev chain rules related to device-mapper devices.
(Run cryptsetup with --debug and you will see that sync point.)

Sometimes it is hidden by the fact that libdevampper could fallback to device internal nodes creation because it verifies that udev nodes were created (the old way used when you compile it without udev support).

Just saying how upstream is designed to work.