Basic Concepts

In the multithreading model, a process consists of one or more threads of control that share a common address space and most other process resources. Threads must acquire and release locks associated with the data they share. If they fail to do so, a data race may ensue--a situation in which a program may produce different results when run repeatedly with the same input.

Data races are easy problems to introduce. Simply accessing a variable without first acquiring the appropriate lock can cause one. Data races are generally very difficult to find. Symptoms generally manifest themselves only if two threads access the improperly protected data at nearly the same time; hence a data race may easily run correctly for months without showing any signs of a problem. It is extremely difficult to exhaustively test all concurrent states of a program for even a simple multithreaded program, so conventional testing and debugging are not an adequate defense against data races.

Most processes share several resources. Operations within the application may require access to more than one of those resources. This means that the operation needs to grab a lock for each of the resources before performing the operation. If different operations use a common set of resources, but the order in which they acquire the locks is inconsistent, there is a potential for deadlock. For example, the simplest case of deadlock occurs when two threads hold locks for different resources and each thread tries to acquire the lock for the resource held by the other thread.