A fix is available
APAR status
Closed as program error.
Error description
MQ Development finds that the abend occurred in CSQISLOG when attempting to get a free ISLH control block from the ISLH QPHB. After loading the head of the freechain, the task was undispatched and in the interval where it was not dispatched storage contraction ran for this QPHB, freeing the extent containing the ISLH that had been at the head of the free chain. When the original task was redispatched it attempted to reference the ISLH as part of removing it from the freechain, and as the storage had now been freemained, this led to the reported abend. * Additional keywords: ABEND0C4 ABENDS0C4 . ABN=0C4-00000011,C=MQ900.910.DMC -CSQISLOG,M=CSQGFRCV, LOC=CSQILPLM.CSQISLOG+00000584
Local fix
Problem summary
**************************************************************** * USERS AFFECTED: All users of IBM MQ for z/OS Version 9 * * Release 1 Modification 0 and Release 2 * * Modification 0. * **************************************************************** * PROBLEM DESCRIPTION: A timing window exists where a task is * * un-dispatched while getting storage for * * an ISLH control block, and local * * storage compression subsequently * * occurs. If storage compression frees * * the underlying QuickCell block, then a * * subsequent reference to the block by * * the task may result in an ABEND0C4. * **************************************************************** Free QuickCell blocks are maintained on a linked list free chain, and blocks are got and freed to the head of the chain. A timing window exists between reading the head pointer, and reading the block forward pointer. If the task is un-dispatched between reading the head pointer and reading the pointer to the next block on the chain, then the QuickCell block may be freed by storage compression before the task is re-dispatched. A subsequent attempt to read the pointer to the next block may result in an 0C4 abend.
Problem conclusion
The logic for getting an ISLH has been changed to reduce the timing window.
Temporary fix
Comments
APAR Information
APAR number
PH40702
Reported component name
IBM MQ Z/OS V9
Reported component ID
5655MQ900
Reported release
100
Status
CLOSED PER
PE
NoPE
HIPER
NoHIPER
Special Attention
NoSpecatt / Xsystem
Submitted date
2021-09-17
Closed date
2022-01-07
Last modified date
2022-09-24
APAR is sysrouted FROM one or more of the following:
APAR is sysrouted TO one or more of the following:
UI78846 UI78847
Modules/Macros
CSQILCHG CSQILINS CSQILVAL CSQIMPU2 CSQISLOG
Fix information
Fixed component name
IBM MQ Z/OS V9
Fixed component ID
5655MQ900
Applicable component levels
Fix is available
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Document Information
Modified date:
24 September 2022