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
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