Patch received from Jörg Mann by email.
Some Debian versions seem to have typos in their udev rules.
As a consequence, /dev/mars/mydata does not disappear when
leaving primary state.
Other consequences might also exist. In an unreproduced case the
/dev/mars/ object was unlinked while the mount was continuously
active, without causing any harm to operations (besides that
xfs_growfs did not work, which led to the discovery of the
mysterious situation).
Probably the udev userspace may require further fixes.
Fortunately, udev-less systems (as those used by me for testing) are
not affected.
Thomas Schoebel-Theuer