Concurrency: Mutual Exclusion and Synchronization Why we need Mutual Exclusion? Classical examples: Bank Transactions:Read Account (A); Compute A = A ;Compute A = A – 500;Write Account (A); Keep A = B all the time A = A + 1;A = 2 * A; B = B + 1;B = 2 * B; Wrong results if mutual exclusion is not enforced!

Disable Interrupt Software Solution Dekker’s Algorithm Peterson’s Algorithm Hardware Solution Disable Interrupt Test & Set Exchange Instruction

Monitors Semaphores is a powerful and flexible tool for enforcing mutual exclusion and for coordinating processes. It may be difficult to produce a correct program and very difficult to debug. The difficulty is that wait and signal operations will be scattered throughout a program and it is not easy to see the overall effect of these operations on the semaphores they affect. A monitor is a programming language construct that provides equivalent functionality to that of semaphores but easier to control. Monitors is implemented in: –Concurrent Pascal –Pascal-Plus –Modula-2, Module-3 –Ada –As a programming library

Monitors (continue) Monitors can lock on any object. One may want to lock all linked list with one lock; or lock a particular linked list by one lock; or lock an element of a particular linked list by one lock. Monitor with Signal –cwait(condition): Suspended execution of the calling process on a condition. The monitor is now available for use by another process. –csignal(condition): Resume execution of some process suspended after a cwait on the same condition. If there are several such processes, choose one of them; if there is no such process, do nothing. A monitor is a software module consisting of one or more procedures, an initialization sequence, and local data. –The local data variables are accessible only by the monitor’s procedures and not by any external procedures. –A process enters the monitor by invoking one of its procedures. –Only one process may be executing in the monitor at a time; any other process that has invoked the monitor is suspended, waiting for the monitor to become available.

Message Passing Two fundamental requirements must be satisfied when processes interact: synchronization and communication. Message passing is a common approach for distributed systems. It is normally provided in the form of a pair of primitives: –send(destination, message) –receive(source, message) Major design characteristics of message systems for Inter-processor Communication and Synchronization: –Synchronization Send : blocking vs. non-blocking Receive : blocking, non-blocking, test for arrival –Addressing: Direct –Send –Receive : explicit vs. implicit Indirect –Static, dynamic, ownership –Format: message content; Length -- fixed vs. variable –Queuing Discipline: FIFO, Priority Driven

Readers/Writers Problem The readers/writers problem is defined as follows: There is a data shared among a number of processes. The data area could be a file, a block of main memory, or even a back of processor registers. There are a number of processes that only read the data (readers) and a number of processes that only write to the data (writers) The following conditions must be satisfied: –1. Any number of readers may read the data at the same time. –2. Only one writer at a time may write to the data. –3. If a writer is writing to the data, no reader may read it. Two observations: –Readers have priority (writers might starve) –Writers have priority