Ramifications of metastability in bit variables explored via Simpson’s 4-slot mechanism
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Protocol descriptions often fail to take metastability into account. Metastability, however, can undermine protocols which depend on shared bits. In this paper a series of increasingly realistic models of bits are developed in CSP to explore the implications of metastability for Simpson’s 4-slot asynchronous communication mechanism. It is shown that the 4-slot mechanism with realistic bit models preserves data-coherence, freshness, and sequencing, and is Lamport-atomic. We demonstrate that metastability can undermine the correctness of protocols demonstrated correct on the assumption that bits are Lamport-safe; furthermore, realistic bit models can demonstrate that protocols are correct which Lamport-safe bit models would suggest were incorrect or impossible.
KeywordsMetastability Simpson’s 4-slot ACM Safe Atomic Wait-free Freshness
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MBDA UK Ltd. and the Bae Systems DCSC funded this research. Our ideas have benefited from conversations with Profs H.R.Simpson and C.B. Jones, Drs. F. Xia and I. Clark, and Mssrs. Eric Campbell and Rod White. We also acknowledge our debt to Prof. John Rushby’s work on the 4-slot in SAL. Finally, we thank the anonymous referees who provided many helpful observations.