Summary: Towards a Necessary and Su cient Condition
for Wait-free Synchronization?
James H. Anderson and Mark Moir
Department of Computer Science, The University of Maryland
College Park, Maryland 20742-3255 U.S.A.
Abstract. We de ne a class of shared objects called snapshot objects,
and give a necessary and su cient condition for the existence of a wait-
free implementation of such objects from atomic registers. Snapshot ob-
jects can be accessed by means of a read operation that returns the entire
object state, or by a set of operations that do not return values. Our con-
dition for the existence of a wait-free implementation requires that for
any pair of operation invocations, either the two invocations commute
or one overwrites the other.
The resilient implementation of shared objects is a subject that has received
much recent attention. A shared object is a data structure that is accessed by a
collection of processes by means of a xed set of operations. An implementation
of a shared object is k-resilient i any process can complete any operation in a
nite number of steps, provided at most k other processes fail undetectably. An