Process Synchronization

Chapter 6: Process Synchronization

Module 6: Process Synchronization
 Background  The Critical-Section Problem  Peterson’s Solution  Synchronization Hardware  Semaphores  Classic Problems of Synchronization  Monitors  Synchronization Examples  Atomic Transactions

Operating System Concepts

6.2

Silberschatz, Galvin and Gagne ©2005

Background
 Concurrent access to shared data may result in data

inconsistency
 Maintaining data consistency requires mechanisms to

ensure the orderly execution of cooperating processes
 Suppose that we wanted to provide a solution to the

consumer-producer problem that fills all the buffers. We can do so by having an integer count that keeps track of the number of full buffers. Initially, count is set to 0. It is incremented by the producer after it produces a new buffer and is decremented by the consumer after it consumes a buffer.

Operating System Concepts

6.3

Silberschatz, Galvin and Gagne ©2005

Producer while (true) /* produce an item and put in nextProduced while (count == BUFFER_SIZE) ; // do nothing buffer [in] = nextProduced; in = (in + 1) % BUFFER_SIZE; count++; }

Operating System Concepts

6.4

Silberschatz, Galvin and Gagne ©2005

Consumer while (1) { while (count == 0) ; // do nothing nextConsumed = buffer[out]; out = (out + 1) % BUFFER_SIZE; count--; /* consume the item in nextConsumed }

Operating System Concepts

6.5

Silberschatz, Galvin and Gagne ©2005

Race Condition


count++ could be implemented as register1 = count register1 = register1 + 1 count = register1 count-- could be implemented as register2 = count register2 = register2 - 1 count = register2 Consider this execution interleaving with “count = 5” initially:





S0: producer execute register1 = count {register1 = 5} S1: producer execute register1 = register1 + 1 {register1 = 6} S2: consumer execute register2 = count {register2 = 5} S3: consumer execute register2 = register2 - 1