Comment # 18 on bug 954908 from Dr. Werner Fink

(In reply to Marius Tomaschewski from comment #16)

Dependency rules are not systemd specific, it is ordinary math, that is to be
exact Directed Acyclic Graph Theory.  This is the same of what physicists mean
with the order of Cause and Effect.

The problem is normally to identify within a dependency chain that it remain
acyclic.

With the list-dependencies command of the systemctyl command it should be
possible to detect the loop:

       list-dependencies NAME
           Shows required and wanted units of the specified unit. If no
           unit is specified, default.target is implied. Target units are
           recursively expanded. When --all is passed, all other units are
           recursively expanded as well.

       --reverse
           Show reverse dependencies between units with list-dependencies,
          i.e. units with dependencies of type Wants= or Requires= on the
          given unit.

       --after, --before
           Show which units are started after or before with
           list-dependencies, respectively.

So it is up on the reporter to show him/her self and us the dependency chains
to be able to search for a loop.