Progress guarantees when composing lock-free objects

  • Authors:
  • Nhan Nguyen Dang;Philippas Tsigas

  • Affiliations:
  • Department of Computer Science and Engineering, Chalmers University of Technology, Gothenburg, Sweden;Department of Computer Science and Engineering, Chalmers University of Technology, Gothenburg, Sweden

  • Venue:
  • Euro-Par'11 Proceedings of the 17th international conference on Parallel processing - Volume Part II
  • Year:
  • 2011

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Abstract

Highly concurrent and reliable data objects are vital for parallel programming. Lock-free shared data objects are highly concurrent and guarantee that at least one operation, from a set of concurrently executed operations, finishes after a finite number of steps regardless of the state of the other operations. Lock-free data objects provide progress guarantees on the object level. In this paper, we first examine the progress guarantees provided by lock-free shared data objects that have been constructed by composing other lock-free data objects. We observe that although lock-free data objects are composable when it comes to linearizability, when it comes to progress guarantees they are not. More specifically we show that when a lock-free data object is used as a component (is shared) by two or more lock-free data objects concurrently, these objects can no longer guarantee lock-free progress. This makes it impossible for programmers to directly compose lock-free data objects and guarantee lock-freedom. To help programmability in concurrent settings, this paper presents a new synchronization mechanism for composing lock-free data objects. The proposed synchronization mechanism provides an interface to be used when calling a lock-free object from other lock-free objects, and guarantees lock-free progress for every object constructed. An experimental evaluation of the performance cost that the new mechanism introduces, as expected, for providing progress guarantees is also presented.