I'm afraid what you are trying is completely wrong.
In case you need a shared file system then you should use a different technology like Manila.
Multiattach in cinder allows multiple VMS to access the same block device data but it does
NEVER provide any mechanism to guarantee consistency at file system level. Popular filesystems
such as xfs never protect concurrent IO from multiple machines and If you mount the filesystem
on the shared disk by multiple vms and write on it concurrently then you'd end up with a corrupted filesystem.
Usually when you have multiattach-ed block devices then you need to implement a mechanism
to prevent concurrent access (eg. Pacemaker)