Mutual exclusion

In computer science, mutual exclusion refers to the requirement of ensuring that no two concurrent processes are in their critical section at the same time; it is a basic requirement in concurrency control, to prevent race conditions. Here, a critical section refers to a period when the process accesses a shared resource, such as shared memory. The requirement of mutual exclusion was first identified and solved by Edsger W. Dijkstra in his seminal 1965 paper titled Solution of a problem in concurrent programming control, and is credited as the first topic in the study of concurrent algorithms.

A simple example of why mutual exclusion is important in practice can be visualized using a singly linked list (See Figure 1). In such a linked list, the removal of a node is done by changing the "next" pointer of the preceding node to point to the subsequent node (e.g., if node i is being removed then the "next" pointer of node i − 1 will be changed to point to node i + 1). In an execution where such a linked list is being shared between multiple processes, two processes may attempt to remove two different nodes simultaneously, resulting in the following problem: let nodes i and i + 1 be the nodes to be removed; furthermore, let neither of them be the head nor the tail; the next pointer of node i − 1 will be changed to point to node i + 1 and the next pointer of node i will be changed to point to node i + 2. Although both removal operations complete successfully, node i + 1 remains in the list since i − 1 was made to point to i + 1, skipping node i (which was the node that reflected the removal of i + 1 by having its next pointer set to i + 2). This can be seen in Figure 1. This problem (normally called a race condition) can be avoided by using the requirement of mutual exclusion to ensure that simultaneous updates to the same part of the list cannot occur.