Are Concurrent Programs That Are Easier to Write Also Easier to Check?

This quote expresses the importance of choosing the right vocabulary to describe and reason about a concept. There is a long and rich history of language proposals for concurrency: semaphores, locks, wait-and-notify, monitors, conditional critical regions, rendezvous, message passing, and transactional memory. Which of these, if any, forms a good vocabulary? What criteria should one apply? My touchstone is the expression of common concurrency patterns. An important criterion for a vocabulary, in my view, is that it is possible to express common patterns in an “obviously correct” manner. The expectation is that simpler solutions are likely to have fewer or more easily detectable errors. An alternative view of this test is that common concurrency patterns should have simple and short proofs of correctness. To this end, consider three common concurrency patterns: a pipeline, a barrier, and a thread pool. The pipeline is pervasive in network processing. Typically, incoming packets are passed through a number of processing stages. This pattern is trivial to describe in terms of message exchanges (Figure 1). The particular style of message exchange is from Hoare’s work on Communicating Sequential Processes (CSP) [2]: in?msg represents a read from channel in to a variable msg, out!msg represents a write of the value of msg to channel out. Each channel has a single reader and writer. Communication is unbuffered and synchronous, in that both reads and writes block until the complementary operation is enabled, at which point the communication takes place. Next, consider a barrier for N processes. All processes have to reach the barrier before any can proceed. In the code in Figure 2, the barrier process counts arrivals; once all processes have arrived, it issues Proceed messages. The alt construct (also from CSP) is used to synchronize with one of toBarrier[1]?Arrived, ..., toBarrier[N]?Arrived. For the final, everyday example, consider a taxi rank at an airport (Figure 3). A taxi driver registers with a dispatcher, she is assigned a passenger, and she