A novel formal theory of concurrent systems is introduced that does not assume any atomic operations. The execution of a concurrent program is modeled as an abstract set of operation executions with two temporal ordering relations: "precedence" and "can causally a#ect". A primitive interprocess communication mechanism is then defined. In Part II, the mutual exclusion is expressed precisely in terms of this model, and solutions using the communication mechanism are given. Contents 1 Introduction 2 2 The Model 2 2.1 Physical Considerations . . . . . . . . . . . . . . . . . . . . . 3 2.2 System Executions . . . . . . . . . . . . . . . . . . . . . . . . 5 2.3 Higher-Level Views . . . . . . . . . . . . . . . . . . . . . . . . 7 3 Interprocess Communication 9 4 Processes 14 5 Multiple-Reader Variables 17 6 Discussion of the Assumptions 18 7 Conclusion 19 1 1 Introduction The mutual exclusion problem was first described and solved by Dijkstra in [3]. In this problem, there is a collection...

Transactional memory (TM) is a promising paradigm for concurrent programming. Whereas the number of TM implementations is growing, however, little research has been conducted to precisely define TM semantics, especially their progress guarantees. This paper is the first to formally define the progress semantics of lockbased TMs, which are considered the most effective in practice. We use our semantics to reduce the problems of reasoning about the correctness and computability power of lock-based TMs to those of simple try-lock objects. More specifically, we prove that checking the progress of any set of transactions accessing an arbitrarily large set of shared variables can be reduced to verifying a simple property of each individual (logical) try-lock used by those transactions. We use this theorem to determine the correctness of state-of-the-art lock-based TMs and highlight various configuration ambiguities. We also prove that lock-based TMs have consensus number 2. This means that, on the one hand, a lock-based TM cannot be implemented using only read-write memory, but, on the other hand, it does not need very powerful instructions such as the commonly used compare-and-swap. We finally use our semantics to formally capture an inherent trade-off in the performance of lock-based TM implementations. Namely, we show that the space complexity of every lock-based software TM implementation that uses invisible reads is at least exponential in the number of objects accessible to transactions.

We propose and solve a synchronization problem called the mailbox problem, motivated by a particular type of interaction between a processor and an external device or between two threads. In this problem, a postman delivers letters to the mailbox of a home owner and uses a flag to signal a non-empty mailbox. The owner must remove all letters delivered to the mailbox and should not walk to the mailbox if it is empty. We present algorithms and an impossibility result for this problem.