You must use sfcumount(8) or sfcumntgl(8) to unmount a GFS Shared File System.
If the GFS Shared File System is unmounted with umount(8) by mistake, the file system cannot be mounted or unmounted. Restoration of such a file system can be performed in the manner described in "C.3.3 Action to Be Taken When Unmounted by umount(8)."
