Hi, I stumbled upon this bug and it took us about a week of debugging with systemd maintainers (thank you very much all of you, particularly michich and kabi_) to figure out the culprit was snoopy.
The deadlock is not happening 100% of the time, probably because snoopy uses a buffer, and systemd's ordering might be very slightly different from one boot to the other. I think it is important to consider backporting this bugfix to debian stable asap, as people use this tool for security reasons. Best regards Loïc
