2421468 - Dual stack system Upgrade/Update - SUM hangs at GETSYNC* or SETSYNC* phase

During update/upgrade of Dual stack system , SUM gets into hang situation in a GETSYNC* or SETSYNC* phase.

Examples of phases where this can occur:

MAIN_SHDIMP/SETSYNC_SHD_FINISHED
MAIN_PROFSTART/SETSYNC_PROFILES_CHANGED
MAIN_SHDPREPUT/SETSYNC_SHD_STARTED_PRE 
MAIN_SHDCRE/SETSYNC_EULCK_START_D1
MAIN_SHDIMP/GETSYNC_PREUP_FINI_D
MAIN_PROFEND/GETSYNC_MODPROF_STARTED 
..etc.

When this occurs, there is no update in logs and no activity being performed. Checking the logs in ABAP tab of SUM you can see below info and in JAVA tab of SUM "Waiting for SUM ABAP event"

 In \SUM\abap\log\SAPup.log

----------------------------------------------------------

CURRENTPHASE MAIN_SHDIMP/SETSYNC_PREUP_FINI_D
...started at 201x0119101504# Setting syncpoint PREUPTIME=FINISHED...finished at 201x0119101505 with status SUCCEEDED.

CURRENTPHASE MAIN_SHDIMP/GETSYNC_PREUP_FINI_D
...started at 201x0119101505
# Checking for syncpoint PREUPTIME=FINISHED
# Java has status "", we'll wait and check every 20 second(s).
# Java has status "", wait another 5 minutes before logging...
# Java has status "", wait another 5 minutes before logging...

# Java has status "", wait another 60 minutes before logging...

 --------------------------------------------------------

 \SUM\abap\log\PHASES.LOG

        ---------------------------------------------------------

1 ETQ200 Executing actual phase 'MAIN_SHDIMP/SETSYNC_SHD_FINISHED'.
1 ETQ399 Phase arguments:
2 ETQ399 Arg[0] = 'SHADOWINST'
2 ETQ399 Arg[1] = 'FINISHED'

-------------------------------------------------------

\SUM\abap\log\SAPupconsole.log

       -------------------------------------------------------

>> 2016/04/05 10:35:40 START OF PHASE MAIN_SHDIMP/SETSYNC_SHD_FINISHED
 ** Remove dead OS process '8536' from internal list.
No items found to display!
No items found to display in 'CHECKS.TXT'! 

>> Current module status: stopped in MAIN_SHDIMP/SETSYNC_SHD_FINISHED
No items found to display in 'CHECKS.TXT'!
>> Waiting for request handler...
>> Received 'next' action for unit 'PRE-EXECUTE'.

=========== Repeat Phase ===========
RESTART: Stopped in phase MAIN_SHDIMP/SETSYNC_SHD_FINISHED

01) * Repeat phase MAIN_SHDIMP/SETSYNC_SHD_FINISHED to continue at the point it stopped
02) - Exit program

: Repeat phase MAIN_SHDIMP/SETSYNC_SHD_FINISHED to continue at the point it stopped

------------------------------------------------------

Under JAVA log directory latest files are updated with the following entry :

Sep 4, 201x 1:49:50 PM [Info ]: Module pre-execute is waiting for event
PREUPTIME=FINISHED
Sep 4, 201x 1:53:59 PM [Info ]: Shared input catalog property
UiTopology(-1, ) set to DUAL_STACK by SLP
Sep 4, 201x 1:53:59 PM [Info ]: Shared input catalog saved to
/upgrade/SUM/sdt/data/SharedInputCatalog.xml
Sep 4, 201x 2:14:38 PM [Info ]: Shared input catalog property
UiTopology(-1, ) set to DUAL_STACK by SLP

• Dual Stack System Upgrade/Update
• Software Update Manager

looping no error no update waiting event solman 7.2 7.1 7.02 , SETSYNC, GETSYNC, SYNC.DAT, SYNC.LCK , KBA , BC-UPG-TLS-TLJ , Upgrade tools for Java , BC-UPG-TLS-TLA , Upgrade tools for ABAP , Problem