Mutual exclusion

In computer science, mutual exclusion is a property of concurrency control, which is instituted for the purpose of preventing race conditions; it is the requirement that one thread of execution never enter its critical section at the same time that another concurrent thread of execution enters its own critical section.

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, which 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 of four items, where the second and third are to be removed. The removal of a node that sits between 2 other nodes is performed by changing the next pointer of the previous node to point to the next node (in other words, if node i is being removed, then the next pointer of node i − 1 is changed to point to node i + 1, thereby removing from the linked list any reference to node i). When such a linked list is being shared between multiple threads of execution, two threads of execution may attempt to remove two different nodes simultaneously, one thread of execution changing the next pointer of node i − 1 to point to node i + 1, while another thread of execution changes the next pointer of node i to point to node i + 2. Although both removal operations complete successfully, the desired state of the linked list is not achieved: node i + 1 remains in the list, because the next pointer of node i − 1 points to node i + 1.

This problem (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.

The problem which mutual exclusion addresses is a problem of resource sharing: how can a software system control multiple processes' access to a shared resource, when each process needs exclusive control of that resource while doing its work? The mutual-exclusion solution to this problem makes the shared resource available only while the process is in a specific code segment called the critical section. It controls access to the shared resource by controlling each process' execution of that part of its program where the resource would be used.

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