Operating System - Deadlock and synchronization

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Deadlock and Synchronization

Among the problems that need to be addressed by computer scientists in order for sophisticated operating systems to be built are deadlock and process synchronization. Deadlock occurs when two or more processes (programs in execution) request the same resources and are allocated them in such a way that a circular chain of processes is formed, where each process is waiting for a resource held by the next process in the chain. As a result, no process can continue; they are deadlocked. An operating system can handle this situation with various prevention or detection and recovery techniques. For example, resources might be numbered 1, 2, 3, and so on. If they must be requested by each process in this order, it is impossible for a circular chain of deadlocked processes to develop. Another approach is simply to allow deadlocks to occur, detect them by examining nonactive processes and the resources they are holding, and break any deadlock by aborting one of the processes in the chain and releasing its resources.

Process synchronization is required when one process must wait for another to complete some operation before proceeding. For example, one process (called a writer) may be writing data to a certain main memory area, while another process (a reader) may be reading data from that area and sending it to the printer. The reader and writer must be synchronized so that the writer does not overwrite existing data with new data until the reader has processed it. Similarly, the reader should not start to read until data has actually been written to the area. Various synchronization techniques have been developed. In one method, the operating system provides special commands that allow one process to signal to the second when it begins and completes its operations, so that the second knows when it may start. In another approach, shared data, along with the code to read or write them, are encapsulated in a protected program module. The operating system then enforces rules of mutual exclusion, which allow only one reader or writer at a time to access the module. Process synchronization may also be supported by an interprocess communication facility, a feature of the operating system that allows processes to send messages to one another.

Designing software as a group of cooperating processes has been made simpler by the concept of "threads." A single process may contain several executable programs (threads) that work together as a coherent whole. One thread might, for example, handle error signals, another might send a message about the error to the user, while a third thread is executing the actual task of the process. Modern operating systems provide management services (e.g., scheduling, synchronization) for such multithreaded processes.