Computer Laboratory

Technical reports

Concurrent verification for sequential programs

John Wickerson

May 2013, 149 pages

This technical report is based on a dissertation submitted December 2012 by the author for the degree of Doctor of Philosophy to the University of Cambridge, Churchill College.

Abstract

This dissertation makes two contributions to the field of software verification. The first explains how verification techniques originally developed for concurrency can be usefully applied to sequential programs. The second describes how sequential programs can be verified using diagrams that have a parallel nature.

The first contribution involves a new treatment of stability in verification methods based on rely-guarantee. When an assertion made in one thread of a concurrent system cannot be invalidated by the actions of other threads, that assertion is said to be ‘stable’. Stability is normally enforced through side-conditions on rely-guarantee proof rules. This dissertation instead proposes to encode stability information into the syntactic form of the assertion. This approach, which we call explicit stabilisation, brings several benefits. First, we empower rely-guarantee with the ability to reason about library code for the first time. Second, when the rely-guarantee method is redeployed in a sequential setting, explicit stabilisation allows more details of a module’s implementation to be hidden when verifying clients. Third, explicit stabilisation brings a more nuanced understanding of the important issue of stability in concurrent and sequential verification; such an understanding grows ever more important as verification techniques grow ever more complex.

The second contribution is a new method of presenting program proofs conducted in separation logic. Building on work by Jules Bean, the ribbon proof is a diagrammatic alternative to the standard ‘proof outline’. By emphasising the structure of a proof, ribbon proofs are intelligible and hence pedagogically useful. Because they contain less redundancy than proof outlines, and allow each proof step to be checked locally, they are highly scalable; this we illustrate with a ribbon proof of the Version 7 Unix memory manager. Where proof outlines are cumbersome to modify, ribbon proofs can be visually manoeuvred to yield proofs of variant programs. We describe the ribbon proof system, prove its soundness and completeness, and outline a prototype tool for mechanically checking the diagrams it produces.

Full text

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BibTeX record

@TechReport{UCAM-CL-TR-834,
  author =	 {Wickerson, John},
  title = 	 {{Concurrent verification for sequential programs}},
  year = 	 2013,
  month = 	 may,
  url = 	 {http://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-834.pdf},
  institution =  {University of Cambridge, Computer Laboratory},
  number = 	 {UCAM-CL-TR-834}
}