PH50945: MQ Z/OS: ABEND5C6-00940001 DUE TO A TIMING ISSUE WHEN USING THE CSQSERVICE1 PARAMETER FOR XA CLIENT CHANNELS

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• The CHIN job log reports an error message:
  CSQX143E CSQXSERV Adapter subtask ended abnormally,
    TCB=nnnnnnnn reason=5C6000-00940001
  
  The abend dump is for:
    ABN=5C6-00940001,U=ssidCHIN,C=MQ900.920.CMC -CSQMCPRH,M=CSQ
    GFRCV,LOC=CSQMCGLM.CSQMCPRH+00005B82
  
  Reg10 from the abend registers points to the ACE control block
  for an instance of a SVRCONN channel that experienced problems
  at the time the abend occurred. The ACE has a pointer to an
  XTE control block (XID Hash Table Entry) that has an
  eyecatcher of FREE. An XID is a conversation ID.
  
  
  Testing has identified two different scenarios that can lead to
  the reported abend.
  
  The first scenario is a timing window where multiple XA_START
  requests are issued for the same XID around the same time.
  There is a timing window where both transactions are allocated
  the same XTE. When one of the transactions completes, the XTE
  will be freed. This can result in the reported abend when one
  of the other transactions makes its next XA request.
  
  The second scenario is a timing window where an XA_END request
  is issued for a transaction in an invalid state. This backs out
  the transaction but does not correctly remove the XTE
  association from the conversation ACE/EB. The next XA request
  for that conversation can result in a reused or freed XTE being
  referenced, leading to the reported abend.
  
  During testing, ABEND0C4 abends in CSQMXTRD were observed
  during QMGR and CHIN shutdown. That module deals with XTE
  control blocks.
  
  These timing problems are exclusive to SVRCONN channels that
  have 'CSQSERVICE1' in the description field for XA
  compatibility mode.
  

Local fix

• N/A
  

Problem summary

• ****************************************************************
  * USERS AFFECTED: All users of IBM MQ for z/OS Version 9       *
  *                 Release 2 Modification 0 and                 *
  *                 Release 3 Modification 0.                    *
  ****************************************************************
  * PROBLEM DESCRIPTION: An 5C6-00940001 abend is seen in the    *
  *                      Channel Initiator address space after   *
  *                      a series of XA api calls.               *
  ****************************************************************
  An 5C6-00940001 abend is seen in the Channel Initiator address
  space after a series of XA api calls.
  
  There are two cases where this can occur:
  
  Case 1 is caused by a timing window where multiple XA_START
  requests are issued for the same XID around the same time and
  both transactions are allocated the same XTE. When one of the
  transactions completes, the XTE is freed causing the reported
  abend from the remaining XA thread.
  
  Case 2 is caused by an XA_END api call request not correctly
  removing the XTE association from the conversation ACE/EB.
  Subsequent XA api calls that reuse the same XTE can lead to the
  reported abend.
  

Problem conclusion

• The code has been corrected so that the timing window for case
  1 can no longer occur and the XTE association is correctly
  cleaned up during XA END processing to fix case 2.
  

Temporary fix

Comments

APAR Information

• APAR is sysrouted FROM one or more of the following:

• APAR is sysrouted TO one or more of the following:

  UI90478 UI90479

Modules/Macros

• CSQMCLMT CSQMXARH
  

Fix information

• Fixed component name

  IBM MQ Z/OS V9

• Fixed component ID

  5655MQ900

Applicable component levels

• R200 PSY UI90479

     UP23/03/08 P F303

• R300 PSY UI90478

     UP23/03/08 P F303

Fix is available

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