PH50960: MQ Z/OS : 0C4-11 ABEND AT CSQILPLM.CSQMHDRS FOR SHARECNV > 0 CHANNELS 22/11/17 PTF PECHANGE

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• The 0C4 abend as below occurred during an MQGET from a queue Q1
  from an async consumer thread for a conversation on a channel
  CHL1. The adapter had retrieved a message from the queue and
  was copying an RFH2 into the async consumer buffer. The abend
  occurred because the buffer had been freed.
  CHINIT trace shows that the buffer was being used by two async
  consumer threads at the same time which is unexpected. When one
  of the threads needed to get a bigger buffer to hold a new
  message, it freed the old buffer. This left the second async
  consumer referencing a now freed buffer.
  The trace shows that the problem occurred when a previous
  conversation freed the same buffer twice. Between the two
  frees, another async consumer thread obtained the buffer which
  resulted in the buffer being on the free chain while being in
  use.
  Other symptoms of this problem are MQGETs failing with
  MQRC_BUFFER_ERROR, incorrect or corrupt messages being returned
  on MQGETs and 0C4 abends in CSQXBPQY. This problem only applies
  to consumers from SHARECNV > 0 channels.
  DUMP TITLE=CSQ1,ABN=0C4-00000011,U=CSQ1CHIN,C=MQ900.9XX.MMC
  -QMHDRS,M=CSQGFRCV,LOC=CSQILPLM.CSQMHDRS
  DUMP TITLE=CSQ1,ABN=0C4-00000011,C=MQ900.9XX.CHIN,
  M=CSQXDISP,LOC=CSQXRCTL.CSQXBPQY
  This problem is a rare timing window and is a regression caused
  by APAR PH44463/PTF UI82256.
  search keyword ABEND0C4
  

Local fix

• n/a
  

Problem summary

• ****************************************************************
  * USERS AFFECTED: All users of IBM MQ for z/OS Version 9       *
  *                 Release 1 Modification 0, Release 2          *
  *                 Modification 0 and Release 3 Modification 0. *
  ****************************************************************
  * PROBLEM DESCRIPTION: A timing window exists whereby multiple *
  *                      channel initiator (CHIN) async consume  *
  *                      threads can obtain the same message     *
  *                      buffer. This scenario can result in 0C4 *
  *                      abends in CSQMHDRS and CSQXBPQY,        *
  *                      MQRC_BUFFER_ERROR being returned on     *
  *                      MQGETs and incorrect or corrupt         *
  *                      messages being returned on MQGETs. This *
  *                      problem will be accompanied by an       *
  *                      earlier CSQX206E message for the        *
  *                      channel.                                *
  *                                                              *
  *                      This problem only applies to consumers  *
  *                      making client connections on SHARECNV > *
  *                      0 SVRCONN channels.                     *
  ****************************************************************
  APAR PH44463 fixed a problem where an async consume thread
  leaked a message buffer. Unfortunately the change introduced a
  timing window where the message buffer could now be freed twice.
  If another async consume thread obtains the message buffer
  between the two frees, then the message buffer will be on a free
  chain while in active use by an async consume thread.
  
  Depending on the order of events which follow, this state can
  lead to a variety of follow on problems. If the buffer is freed
  back to the system from the free chain, then the async consume
  thread can experience 0C4 abends when using the buffer. If
  instead a second async consume thread obtains the buffer from
  the free chain, then this can result in either thread returning
  corrupt or invalid messages to the client on MQGET calls.
  

Problem conclusion

• The channel initiator async consume thread logic which frees
  message buffers has been corrected to only free a message buffer
  once.
  

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:

  UI83602 UI83603 UI83604

Modules/Macros

• CSQXRSTM
  

Fix information

• Fixed component name

  IBM MQ Z/OS V9

• Fixed component ID

  5655MQ900

Applicable component levels

• R100 PSY UI83604

     UP22/12/23 P F212 ­

• R200 PSY UI83603

     UP22/12/23 P F212 ­

• R300 PSY UI83602

     UP22/12/23 P F212 ­

Fix is available

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