In systemd prior to 234 a race exists between .mount and .automount units such that automount requests from kernel may not be serviced by systemd resulting in kernel holding the mountpoint and any processes that try to use said mount will hang. A race like this may lead to denial of service, until mount points are unmounted.
An upstream issue:
An upstream patch:
Created systemd tracking bugs for this issue:
Affects: fedora-all [bug 1535130]
This issue has been addressed in the following products:
Red Hat Enterprise Linux 7
Via RHSA-2018:0260 https://access.redhat.com/errata/RHSA-2018:0260