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Acquisition of a Lock while Holding a Lock

IP.com Disclosure Number: IPCOM000106454D
Original Publication Date: 1993-Nov-01
Included in the Prior Art Database: 2005-Mar-21
Document File: 2 page(s) / 60K

Publishing Venue

IBM

Related People

Rechtschaffen, R: AUTHOR

Abstract

In multiprocessing systems the operation within the memory hierarchy can be conditioned by the fact that the processor has acquired and not yet released a lock-word. The acquisition of a second lock is a case in point that should affect the processor's subsequent actions.

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Acquisition of a Lock while Holding a Lock

      In multiprocessing systems the operation within the memory
hierarchy can be conditioned by the fact that the processor has
acquired and not yet released a lock-word.  The acquisition of a
second lock is a case in point that should affect the processor's
subsequent actions.

      Let us consider a Multiprocessing System that is implemented
with a memory hierarchy that is made up of a WI (Write In) L2 cache
and WTWAX (Write Through Write Allocate Exclusive) L1 caches.

      The problem with such a memory hierarchy concerns the exclusive
status of data following the final store to such data and the
reluctance to acquire data on an exclusive basis unnecessarily.  The
acquisition and retention of a lock-word changes this situation
especially if the release of the lock-word will relinquish the
exclusive status of all lines acquired while the lock was held.

      The acquistion of a second lock while the first lock is held is
a case in point.  The acquisition of locks has a special feature in
that lock-acquisition must contend with deadlock avoidance.  Thus
locks are acquired relative to a lock hierarchy that the software
must enforce.  If this rule is violated no hardware action can
restore the proper order to the system.

      Several meaningful prompts to the proper management of
EXCLUSIVITY can be derived from the lock hierarchy.  The pattern of
usage of COMPARE & SWAP instructions against memory locations within
a multiprocessor system can be used to select those COMPARE & SWAP
instructions that take a LOCK and releas...