PI15999: REBUILD SCA CAUSED SYSTEM TO HANG FROM RL LATCH CONTENTION BETWEEN DDF DBAT THREAD IN COMMIT AND SCA REBUILD SERVICE TASK

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• DB2DDF DB2D2PC DB2XA DB2DSHR DB2TERM defect pi15999 dpi15999
  Customer issues the SETXCF START,REBUILD,STRNAME=xxx_SCA
  command and the system hangs.  At the time of the command
  issuance, a DDF online transaction is going through commit
  processing and holding RL latch. DB2 service task that
  supports the SCA rebuild also is suspended waiting for
  the RL latch.  The DDF dbat thread is also waiting for the
  same DB2 service task to complete, thus the hang.
  DB2 needs to be recycled to break the hang.
  ***************************************************************
  Additional Symptoms and Keywords:
   ABEND026 AB026 S0026 026
    026-08118001 08118001 RC08118001
   RLLATCH SCA Rebuild
  

Local fix

Problem summary

• ****************************************************************
  * USERS AFFECTED: All Distributed Data Facility (DDF) users.   *
  *                 Specifically where DB2 is a member of a      *
  *                 data sharing group.                          *
  *                 DB2 11 for z/OS users only.                  *
  ****************************************************************
  * PROBLEM DESCRIPTION: Deadlock between SCA Rebuild and an     *
  *                      XA two phase commit sequence from a     *
  *                      remote system.                          *
  *                      The SCA Rebuild deadlock condition      *
  *                      results in periodic abend 026-08118001  *
  *                      conditions.                             *
  ****************************************************************
  * RECOMMENDATION:                                              *
  ****************************************************************
  In order to support remote client XA transaction managers when
  DB2 is a member of a data sharing group, potential indoubt XID
  information must be recorded in the SCA since the remote XA
  client may contact ANY member of the group in order to obtain
  indoubt XID information related to ALL members.
  As a result, during DRDA XA Phase 1 Commit processing,
  xa_prepare(), the DB2 z/OS server will add an "xid" entry to
  the SCA.  Once a (commit/abort) decision is received, DB2 will
  remove the "xid" entry from the SCA.
  In DB2 11 for z/OS, DB2 DDF is now involved in SCA Rebuild
  processing.  A deadlock condition may occur if an SCA Rebuild
  is initiated while DB2 (DDF) is able to service DRDA XA
  transactions.
  The deadlock occurs because SCA Rebuild processing must wait
  for a latch that is held by another thread that is processing
  the XA transaction decision.  However, the XA transaction
  decision requires that the associated entry be deleted from the
  SCA but this waits due to the SCA Rebuild that is underway.
  This deadlock condition results in the periodic detection of
  abend 026-08118001.  The DB2 subsystem must be terminated to
  break the deadlock condition.
  

Problem conclusion

• DB2 DDF SCA entry delete processing has been changed to
  eliminate the serialization issue that can lead to the deadlock,
  or to conditionally delete the SCA entry since (if an SCA
  Rebuild is underway) there is no need to delete the SCA entry
  because the SCA Rebuild will reflect the completion of the XA
  transaction.
  

Temporary fix

• *********
  * HIPER *
  *********
  

Comments

APAR Information

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

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

  UI18017

Modules/Macros

• DSNLCMT2 DSNLTBAB DSNLTRD  DSNLTRSI DSN7LDE1
  

Fix information

• Fixed component name

  DB2 OS/390 & Z/

• Fixed component ID

  5740XYR00

Applicable component levels

• RB10 PSY UI18017

     UP14/05/30 P F405 ½

Fix is available

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