https://bugs.freedesktop.org/show_bug.cgi?id=107154
--- Comment #11 from kyle.devir@mykolab.com ---
systemd-udevd
This is not systemd's fault, but indicative of something hanging in kernel land, which udevd ends up being blocked on.
Experienced this a few major kernel releases ago, which were resolved by the next major version. Never did figure out what caused udevd to block... :/