Exceptions and side-effects in atomic blocks

In recent work we showed how to implement a new atomic keyword as an extension to the Java programming language. It allows a program to perform a series of heap accesses atomically without needing to use mutual exclusion locks. We showed that data structures built using it could perform well and scale to large multi-processor systems. In this paper we extend our system in two ways. Firstly, we show how to provide an explicit 'abort' operation to abandon execution of an atomic block and to automatically undo any updates made within it. Secondly, we show how to perform external I/O within an atomic block. During our work we found that it was surprisingly difficult to support these operations without opening loopholes through which the programmer could subvert language-based security mechanisms. Our final design is based on a 'external action' abstraction, allowing code running within an atomic block to request that a given pre-registered operation be executed outside the block.