Corrective action

Take an SVC dump of all systems in the sysplex. Include the OMVS address space and all OMVS data spaces in the dump. See Getting the right z/OS UNIX data in a dump to determine what DUMP parameters to use. To initiate the dump on all systems, use the REMOTE=(SYSLIST=(system1,system2,...),SDATA,DSPNAME) parameter. For more information on the DUMP system command, see z/OS MVS System Commands.
Note that FIX processing performed in the next step also initiates an SVC dump, but the dump includes different data, capturing critical global file system resources.

Retain all dumps in case you need to provide them to the IBM® Service Center for analysis.
Issue the MODIFY BPXOINIT,FILESYS=FIX system command. The system should return message BPXF049; it lists the systems that are causing unmount processing delay. Message BPXF042I also appears for each system that has contention for the file system MOUNT latch. Contention for the MOUNT latch delays high-level functions, such as mount and unmount processing. Finally, message BPXF057I appears for each file system that has latch contention.
On each system for which FIX has reported latch contention, issue the D GRS,LATCH,C system command to determine if latch contention still exists on the system. If latch contention still exists, restart the system. After partition recovery has completed on the restarted system, repeat Step 3 on the next identified system.
If unmount processing delay continues, return to Step 2, then repeat step 3 for any systems identified as having possible latch contention. Repeat Step 2 and Step 3 to verify that no latch contention exists.
If unmount processing delay continues, and FIX, in message BPXF049I, identified systems that owe responses, restart the identified systems.
If FIX does not identify any other systems as owing responses, then issue the MODIFY BPXOINIT,FILESYS=UNMOUNT,FILESYSTEM=file system name system command to unmount the file system.