ISGCNFXITSYSTEM — Filter Global Resource Serialization Contention Notification, SYSTEM Scope

ISGCNFXITSYSTEM provides a way for your installation to suppress the ENF 51 signal that global resource serialization issues to notify programs of contention for a scope=SYSTEM resource. When at least one exit routine has been added to the ISGCNFXITSYSTEM exit, it will receive control for every resource of system scope that Global Resource Serialization finds in contention. Depending on what your installation specified in the exit routine, the exit can suppress the ENF 51 signal for the resource in contention. This allows the installation to suppress ENF signals for resources known to be frequently in contention, with the trade-off of providing less data to any monitoring tools that listen for those signals.

Replacing the Exit Routine

For information regarding dynamic exit routine replacement, see Replacing a Dynamic Exit Routine.

Exit Routine Environment

ISGCNFXITSYSTEM receives control in the following environment:

Enabled for interrupts.
In supervisor state with PSW key 0.
In primary ASC mode with H=S=requestor's address space, and P=Global Resource Serialization's address space.
In task mode.
In AMODE 31 and RMODE ANY.
With no locks held, a local lock held, or both a local lock and the CMSEQDQ lock held..

Exit Recovery is as follows.

If an error occurs, ISGCNFXITSYSTEM provides with its own recovery routine.
If no recovery exists, or the recovery continues with termination, GRS fails the request and continues processing. CSVDYNEX rules for the exit are used to determine whether the exit should be removed.
The unplanned removal of an exit can result in serialization changes. Such changes might cause data integrity errors. Therefore, it is important to provide recovery and to understand how CSVDYNEX determines when to remove an abend exit. For more information about CSVDYNEX, see z/OS MVS Programming: Authorized Assembler Services Reference ALE-DYN.

Exit Routine Processing

The ISGCNFXITSYSTEM exit point allows an exit routine to suppress the ENF 51 signal when global resource serialization detects contention on a system scope resource. The exit routine is passed a parameter list, CNFP (mapped by CNFP), which contains information about the request. The exit routine can check the QNAME, RNAME, and Rnamelen of the resource and filter the ENF 51 signal.

Programming Considerations

Observe the following considerations when coding an exit routine to filter global resource seriaqlization contention notification on a system scope:

Every exit routine must be reentrant.
While coding the exit routine, be aware that you will have an increased path length for ENQ/DEQ requests because the exit is called for every system scope resource in contention.
Depending on the locks held upon entry to the exit routine, failing to return control promptly can cause significant delays in global resource serialization processing.
Make sure that your exit routine takes into account the parameter list version, CNFP_VERSION, in case of future interface changes for the exit point.
Do not code the exit routine to change any variables in the parameter list except for the filter flag. The exit does not make a copy of the resource identifying parameters passed, such as QNAME, RNAME, and Rnamelen.
If you want to write an exit routine that covers both system (ISGCNFXITSYSTEM) and systems (ISGCNFXITSYSPLEX) scope resource contention, then use the CNFP_SYSTEM flag in the CNFP parameter list to differentiate between them.
The resource described in the parameter list via CNFP_QNAME@, CNFP_RNAME@, CNFP_RLEN, and CNFP_SYSTEM reflects any changes made from other exits or RNL processing.

Entry Specifications

The system passes a parameter list (CNFP) to the exit routine.

Registers at Entry: The contents of the registers on entry to the exit are as follows.

Register: Contents
0: Not applicable
1: Address of the exit parameter list (ISGYCNFP)
2-12: Not applicable
13: Address of a 72-byte save area
14: Return address
15: Entry point of ISGCNFXITSYSTEM

Parameter List Contents: Register 1 contains the address of the exit parameter (CNFP) that is mapped by macro ISGYCNFP. The parameter list contains the QNAME, RNAME, and Rnamelen of the resource, the filter flag, and a 4K work area. You should not use this work area to communicate among multiple exit routines because the work area may not be cleared upon entry. In addition, in certain recovery scenarios the work area may not be the same for all routines.

Return Specifications

At the completion of ISGCNFXITSYSTEM processing, ISGYCNFP can indicate that the ENF 51 signal be suppressed by setting the CNFP_FILTER flag.

Registers at Exit: Upon return from the exit processing, the register contents must be as follows.

Register: Contents
0-14: Restored to contents on entry
15: 0

Coded Example of the Exit Routine

IBM® provides coded examples of the ISGCNFXITSYSTEM and ISGCNFXITSYSPLEX exit routines in member ISGCNFXR of SYS1.SAMPLIB.