For whom it may concern, I had recently updated packages on a Centos 7 test machine. Among many packages updated was: libudisks2-2.7.3-9.el7.x86_64 => 2.8.4-1.el7.x86_64 Related or not, after this upgrade xfstest started failing on failures to unmount. This was especially tests that mounted loop or dm targets, such as xfs/078 and xfs/311. And no, UDEV_SETTLE_PROG did not help, neither did an extra 1s delay. In the end, I bisected this down to disabling udisks2 service. Just posting in case somebody else gets hit by this. Thanks, Amir.
