Despite years of research, the design of efficient nonblocking algorithms remains difficult. A key reason is that current shared-memory multiprocessor architectures support only single-location synchronisation primitives such as compare-and-swap (CAS) and load-linked/store-conditional (LL/SC). Recently researchers have investigated the utility of double-compare-and-swap (DCAS)--a generalisation of CAS that supports atomic access to two memory locations -- in overcoming these problems. We summarise recent research in this direction and present a detailed case study concerning a previously published nonblocking DCAS-based double-ended queue implementation. Our summary and case study clearly show that DCAS does not provide a silver bullet for nonblocking synchronisation. That is, it does not make the design and verification of even mundane nonblocking data structures with desirable properties easy. Therefore, our position is that while slightly more powerful synchronisation primitives can ave a profound effect on ease of algorithm design and verification, DCAS does not provide sufficient additional power over CAS to justify supporting it in hardware.

Assuming the existence of garbage collection makes it easier to design implementations of concurrent data structures. However, this assumption limits their applicability. We present a methodology that, for a significant class of data structures, allows designers to first tackle the easier problem of designing a garbage-collection-dependent implementation, and then apply our methodology to achieve a garbage-collection-independent one. Our methodology is based on the well-known reference counting technique, and employs the double compare-and-swap operation.

The computer industry is currently examining the use of strong synchronization operations such as double compare-and-swap (DCAS) as a means of supporting non-blocking synchronization on tomorrow's multiprocessor machines. However, before such a strong primitive will be incorporated into hardware design, its utility needs to be proven by developing a body of effective non-blocking data structures using DCAS. As part of this effort, we present two new linearizable non-blocking implementations of concurrent deques using the DCAS operation. The first uses an array representation, and improves on former algorithms by allowing uninterrupted concurrent access to both ends of the deque while correctly handling the difficult boundary cases when the deque is empty or full. The second uses a linked-list representation, and is the first non-blocking unbounded-memory deque implementation. It too allows uninterrupted concurrent access to both ends of the deque.