Concurrent Threads

Initially v=0. Thread 1: for (i=0; i<10; i++) v = v+1; Thread 2: for (i=0; i<10; i++) v = v-1; Concurrent Threads What are the possible values of v at the end? Cpr E 308, Fall 2003

Mutual Exclusion so far Four conditions to provide mutual exclusion • No two processes simultaneously in critical region • No assumptions made about speeds of CPUs • No process running outside its critical region may block another process • No process must wait forever to enter its critical region Cpr E 308, Fall 2003

Solutions to Mutual Exclusion • Dekker’s Algorithm – 1960’s • Peterson’s Algorithm – 1981 • Hardware support Cpr E 308, Fall 2003

Peterson’s Solution Cpr E 308, Fall 2003

Hardware Support with TSL • Instruction TSL R, LockTSL = “Test and Set Lock”R = register, Lock = memory location • Atomic = “all or nothing” • Atomically- reads Lock into R and - Store a non-zero value into Lock Cpr E 308, Fall 2003

Mutual Exclusion using TSL • Problems: • Busy Waiting, • Not very useful on a uniprocessor Cpr E 308, Fall 2003

Eliminate Busy Waiting Implementation of mutex_lock and mutex_unlock Without busy waiting Cpr E 308, Fall 2003

Producer - Consumer • Producer - Insert produced objects into bounder buffer • Consumer - Delete objects from the buffer and eat them • Synchronization required - Exclusive access to the buffer- Buffer full? - Buffer empty? Cpr E 308, Fall 2003

Concurrent Threads