At 07:09 -0700 on 08/01/2006, Edward Jaffe wrote about Re: Data set
ENQueues and DEQueues in Jobs:
Paul Gilmartin wrote:
Likewise, if RET=CHNG were not available, you could equally well
simply DEQ and re-ENQ with EXCL scope.
No. Holding a shared ENQ prevents others from acquiring an exclusive
ENQ on the same resource and modifying it. To maintain the integrity
of the resource, you use RET=CHNG to upgrade from shared to
exclusive without losing control. If you were required to DEQ and
then re-ENQ to perform the upgrade, you would lose control of the
resource between the time you inspected it and the time you had the
necessary serialization to update it. In the worst case scenario,
someone else could change (or even delete) the resource in-between!!
If there are others with a SHARED ENQ, attempting a RET=CHNG will
either put you into a wait (possibly triggering a deadly embrace if
you hold another SHR or EXC ENQ that some other task wants to go to
EXC status on) or return a failure RC and require you to recover by
attempting it again. It is MUCH better to get the EXC initially and
later drop back to SHR (assuming that once you have done the code
that needs the EXC you can live with just read access) but IBM has
not provided this useful capability.
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