An assertional criterion for atomicity

A criterion is presented to prove atomicity of read-write objects by means of ghost variables and invariants. The criterion is applied to Bloom's construction of a two-writer atomic register from two one-writer atomic registers and to the algorithm of Vitanyi and Awerbuch for the construction of a read-write object with m readers and writers, based on m(2) read-write objects for one reader and one writer. In both cases, the proof comes down to the verification of a number of invariants. The hand-written proofs of these invariants have been verified with a mechanical theorem prover.