1. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H بسم الله الرحمن الرحيم Chapter 6: Process Synchronization CPCS361 – Operating Systems I

2. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 2 Module 6: Process Synchronization  Background  The Critical-Section Problem  Peterson’s Solution  Synchronization Hardware  Semaphores  Classic Problems of Synchronization  Monitors  Synchronization Examples  Atomic Transactions

3. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 3 Objectives  To introduce the critical-section problem, whose solutions can be used to ensure the consistency of shared data  To present both software and hardware solutions of the critical-section problem  To introduce the concept of an atomic transaction and describe mechanisms to ensure atomicity

4. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 4 Background  Cooperating processes can either directly share a logical address space (i.e. both code and data) or be allowed to share data only through files or messages  Concurrent access to shared data may result in data inconsistency  Maintaining data consistency requires mechanisms to ensure the orderly execution of cooperating processes

5. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 5 Background  Suppose that we wanted to provide a solution to the consumer-producer problem that fills all the buffers. This can be done 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

6. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 6 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++; }

7. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 7 Consumer while (true) { while (count == 0) ; // do nothing nextConsumed = buffer[out]; out = (out + 1) % BUFFER_SIZE; count--; /* consume the item in nextConsumed */ }

8. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 8 Race Condition  Producer and consumer routines are correct separately they may not function correctly when executed concurrently  Example: count = 5; concurrently: producer executes count++; consumer executes count--; count may be 4, 5, or 6! The only correct result is 5, if producer and consumer execute separately

9. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 9 Race Condition  count++count--  count++ and count-- could be implemented as register1 = count register1 = register1 + 1 count = register1 register2 = count register2 = register2 - 1 count = register2

10. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 10 Race Condition Consider this execution interleaving with “count = 5” initially: S 0 : producer register 1 = count {register 1 = 5} S 1 : producer register 1 = register 1 + 1 {register 1 = 6} S 2 : consumer register 2 = count {register 2 = 5} S 3 : consumer register 2 = register 2 - 1 {register 2 = 4} Then, either: S 4 : producer count = register 1 {count = 6 } S 5 : consumer count = register 2 {count = 4} or: S 4 : consumer count = register 2 {count = 4} S 5 : producer count = register 1 {count = 6 } register1 = count register1 = register1 + 1 count = register1 register2 = count register2 = register2 - 1 count = register2

11. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 11 Race Condition  Incorrect state of count, because it is allowed for both processes to manipulate the variable count concurrently; this is called race condition.  Process synchronization and coordination is needed. register1 = count register1 = register1 + 1 count = register1 register2 = count register2 = register2 - 1 count = register2

12. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 12 Critical-Section Problem  Critical section: a process segment of code in which a process may be changing common variables, updating a table, writing a file, and so on  When one process is executing in its critical section, no other process is allowed to execute in its critical section do { entry section critical section exit section remainder section } while (TRUE);

13. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 13 Solution to Critical-Section Problem  A solution to the critical-section problem must satisfy the following three requirements: 1. Mutual Exclusion - If process P i is executing in its critical section, then no other processes can be executing in their critical sections 2. Progress - If no process is executing in its critical section and there exist some processes that wish to enter their critical section, then only those processes that are not executing in their remainder sections can participate in the decision on which will enter its critical section next, and this selection cannot be postponed indefinitely

14. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 14 Solution to Critical-Section Problem 3. Bounded Waiting - A bound, or limit must exist on the number of times that other processes are allowed to enter their critical sections after a process has made a request to enter its critical section and before that request is granted Assume that each process executes at a nonzero speed No assumption concerning relative speed of the N processes

15. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 15 Peterson’s Solution  Solution is restricted to two processes that alternate execution between their critical sections and reminder sections

16. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 16 Peterson’s Solution  The two processes share two variables:  int turn;  boolean flag[2]  The variable turn indicates whose turn it is to enter the critical section.  The flag array is used to indicate if a process is ready to enter the critical section. flag[i] = true implies that process P i is ready!  If both processes try to enter at the same time, turn will be set to both i and j at roughly the same time. Only one of these will last; the other will occur but will be overwritten immediately  The eventual value of turn decides which of the two processes is allowed to enter its critical section first

17. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 17 do { flag[i] = TRUE; turn = j; while (flag[j] && turn == j); critical section flag[i] = FALSE; remainder section } while (TRUE); Algorithm for Processes P i and P j PiPi do { flag[j] = TRUE; turn = i; while (flag[i] && turn == i); critical section flag[j] = FALSE; remainder section } while (TRUE); PjPj

18. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 18 Algorithm for Processes P i and P j  Mutex is preserved: if P i and P j can be executing in their critical section at the same time, then flag[i] = flag[j] = true. Since turn can be either i or j one of the processes must have successfully executing the while statement  P i will reset flag[i] to false allowing P j to enter its critical section (progress) after at most one entry by P i (bounded waiting)

19. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 19 Synchronization Hardware  Many systems provide hardware support for critical section code  Uniprocessors – could disable interrupts  Currently running code would execute without preemption  Generally too inefficient on multiprocessor systems  Modern machines provide special atomic hardware instructions  Atomic = non-interruptable  Either test memory word and set value  Or swap contents of two memory words

20. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 20 Synchronization Hardware  Race conditions are prevented by requiring that critical regions be protected by locks  A process must acquire a lock before entering a critical section; it releases the lock when it exits the critical section

21. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 21 Solution to Critical-section Problem Using Locks do { acquire lock critical section release lock remainder section } while (TRUE);

22. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 22 Semaphore  Semaphore  S – integer variable  Less complicated  Accessed only through two standard atomic /indivisible operations (when one process modify the semaphore value, no other process can simultaneously modify that same semaphore):  wait() and signal()  Originally called P() and V() wait (S) { while S <= 0 ; // no-op S--; } signal (S) { S++; }

23. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 23 Semaphore as General Synchronization Tool  Counting semaphore – integer value can range over an unrestricted domain  Binary semaphore – integer value can range only between 0 and 1; can be simpler to implement  Also known as mutex locks  Can implement a counting semaphore S as a binary semaphore  Provides mutual exclusion Semaphore mutex; // initialized to 1 do { wait (mutex); // Critical Section signal (mutex); // remainder section } while (TRUE);

24. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 24 Deadlock and Starvation  Deadlock – two or more processes are waiting indefinitely for an event that can be caused by only one of the waiting processes  Let S and Q be two semaphores initialized to 1... P0P0 P1P1 wait (S); wait (Q); wait (S); signal (S); signal (Q); signal (S); can not be executed  deadlock

25. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 25 Deadlock and Starvation  Starvation – indefinite blocking. A process may never be removed from the semaphore queue in which it is suspended. It may occur when processes are added or removed from the semaphore’s list in LIFO order.

26. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 26 Classical Problems of Synchronization 1. Bounded-Buffer Problem 2. Readers and Writers Problem 3. Dining-Philosophers Problem

27. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 27 Bounded-Buffer Problem  Solution to the bounded-buffer problem using semaphores  Pool of n buffers, each can hold one item  Semaphore mutex initialized to the value 1  Semaphore full initialized to the value 0  Semaphore empty initialized to the value n

28. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 28 Bounded Buffer Problem (Cont.) Structure of the producer process do { // produce an item in nextp wait (empty); wait (mutex); // add the item to the buffer signal (mutex); signal (full); } while (TRUE); Structure of the consumer process do { wait (full); wait (mutex); // remove an item from buffer to // nextc signal (mutex); signal (empty); // consume the item in nextc } while (TRUE);

29. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 29 Readers-Writers Problem  A data set is shared among a number of concurrent processes  Readers – only read the data set; they do not perform any updates  Writers – can update (to read and write)  Problem – Allow multiple readers to read at the same time. Only one single writer can access the shared data at the same time (writers have exclusive access to the shared data). No reader will be kept waiting unless a writer has already obtained permission to use the shared object.

30. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 30 Readers-Writers Problem Solution: Shared Data  Integer readcount initialized to 0  keeps track of how many processes are currently reading the object  Semaphore mutex initialized to 1  to ensure mutual exclusion when the variable readcount is updated  Semaphore wrt initialized to 1  mutual exclusion for the writers  used by the first or last reader that enters or exits the critical section

31. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 31 Readers-Writers Problem (Cont.) Structure of a writer process do { wait (wrt) ; // writing is performed signal (wrt) ; } while (TRUE); Structure of a reader process do { wait (mutex) ; readcount ++ ; if (readcount == 1) wait (wrt) ; signal (mutex); // reading is performed wait (mutex) ; readcount --; if (readcount == 0) signal (wrt) ; signal (mutex) ; } while (TRUE);

32. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 32 Dining-Philosophers Problem  Shared data  Bowl of rice (data set)  Semaphore chopstick [5] initialized to 1 Eat Think

33. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 33 Dining-Philosophers Problem (Cont.)  The structure of Philosopher i : do { wait ( chopstick[i] ); wait ( chopstick[ (i + 1) % 5] ); // eat signal ( chopstick[i] ); signal (chopstick[ (i + 1) % 5] ); // think } while (TRUE);

34. Adopted from and based on Textbook: Operating System Concepts – 8th Edition, by Silberschatz, Galvin and Gagne Updated and Modified by Dr. Abdullah Basuhail, CSD, FCIT, KAU, 1431H 34 End of Chapter 6