1 Lecture 8: Concurrency: Mutual Exclusion and Synchronization Advanced Operating System Fall 2012

2 Concurrency An OS has many concurrent processes that run in parallel but share common access Race Condition: A situation where several processes access and manipulate the same data concurrently and the outcome of the execution depends on the particular order in which the access takes place.

3 Example for Race condition Suppose a customer wants to book a seat on UAL 56. Ticket agent will check the #-of-seats. If it is greater than 0, he will grab a seat and decrement #-of-seats by 1. UAL 56: #-of-seats=12 Main memory Terminal … Ticket Agent 1 Ticket Agent 2 Ticket Agent n

4 Example for Race condition(cont.) Ticket Agent 1 P1: LOAD #-of-seats P2: DEC 1 P3: STORE #-of-seats Ticket Agent 2 Q1: LOAD #-of-seats Q2: DEC 1 Q3: STORE #-of-seats Ticket Agent 3 R1: LOAD #-of-seats R2: DEC 1 R3: STORE #-of-seats Suppose, initially, #-of-seats=12 Suppose instructions are interleaved as P1,Q1,R1,P2,Q2,R2,P3,Q3,R3 The result would be #-of-seats=11, instead of 9 To solve the above problem, we must make sure that: P1,P2,P3 must be completely executed before we execute Q1 or R1, or Q1,Q2,Q3 must be completely executed before we execute P1 or R1, or R1,R2,R3 must be completely executed before we execute P1 or Q1.

5 Critical Section Problem Goal: To program the processes so that, at any moment of time, at most one of the processes is in its critical section. Prefix 0 CS 0 Suffix 0 P0P0 Prefix 1 CS 1 Suffix 1 P1P1 Prefix n-1 CS n-1 Suffix n-1 P n-1 … Critical section: a segment of code in which the process may be changing common variables, updating a table, writing a file, and so on.

6 Solution to Critical-Section Problem Any facility to provide support for mutual exclusion should meet the following requirements: 1. Mutual exclusion must be enforced: Only one process at a time is allowed into its critical section 2. A process that halts in its noncritical section must do so without interfering with other processes. 3. A process waiting to enter its critical section cannot be delayed infinitely 4. When no process is in a critical section, any process that requests entry to its critical section must be permitted to enter without delay. 5. No assumption are made about the relative process speeds or the number of processors. 6. A process remains inside its critical section for a finite time only.

7 Three Environments 1. There is no central program to coordinate the processes. The processes communicate with each other through global variable. 2. Special hardware instructions 3. There is a central program to coordinate the processes.

8 Three Environments 1. There is no central program to coordinate the processes. The processes communicate with each other through global variable. 2. Special hardware instructions 3. There is a central program to coordinate the processes.

9 1 st Attempt Start with just 2 processes, P 0 and p 1 Global variable turn, initially turn=0 Prefix 0 While (turn  0) do {} CS 0 turn=1 suffix 0 Prefix 1 While (turn  1) do {} CS 1 turn=0 suffix 1 The processes take turn to enter its critical section If turn=0, P 0 enters If turn=1, P 1 enters This solution guarantees mutual exclusion. But the drawback is that the method is not fair, because P0 is priviledged. Worse yet, until P0 executed its CS, P1 is blocked.

10 2 st Attempt Global variable flag[0] and flag[1], initially flag[0] and flag[1] are both false Prefix 0 While (flag[1]) do {} flag[0]=true CS 0 flag[0]=false suffix 0 Prefix 1 While (flag[0]) do {} flag[1]=true CS 1 flag[1]= false suffix 1 If P 0 is in critical section, flag[0] is true; If P 1 is in critical section, flag[1] is true If one process leaves the system, it will not block the other process. However, mutual exclusion is not guaranteed. P 0 executes the while statement and finds that flag[1] is false; P 1 executes the while statement and finds that flag[0] is false. P 0 sets flag[0] to true and enters its critical section; P 1 sets flag[1] to true and enters its critical section.

11 3 st Attempt Global variable flag[0] and flag[1], initially flag[0] and flag[1] are both false Prefix 0 flag[0]=true While (flag[1]) do {} CS 0 flag[0]=false suffix 0 Prefix 1 flag[1]=true While (flag[0]) do {} CS 1 flag[1]= false suffix 1 If P 0 is in critical section, flag[0] is true; If P 1 is in critical section, flag[1] is true Guarantees mutual exclusion. But mutual blocking can occur. P 0 sets flag[0] to be true; P 1 sets flag[1] to be true; Both will be hung in the while loop.

12 4 st Attempt Global variable flag[0] and flag[1], initially flag[0] and flag[1] are both false Prefix 0 L0: flag[0]=true If (flag[1]) then { flag[0]=false; goto L0} CS 0 flag[0]=false suffix 0 Prefix 1 L1: flag[1]=true If (flag[0]) then { flag[1]=false; goto L1}} CS 1 flag[1]= false suffix 1 Guarantees mutual exclusion. mutual blocking can occur if they execute at the same speed.

13 Correct Solution (Dekker’s Alg) – The first correct mutual exclusion alg (early 1960’s) Initially, flag[0]=flag[1]=false; turn=0 Prefix 0 flag[0]=true while (flag[1]) do {if (turn=1){ flag[0]=false; while(turn=1) do{} flag[0]=true; } CS 0 turn=1 flag[0]=false suffix 0 Prefix 1 flag[1]=true while (flag[0]) do {if (turn=0){ flag[1]=false; while(turn=0) do{} flag[1]=true; } CS 1 turn=0 flag[1]=false suffix 1

14 Peterson’s Algorithm for 2 processes – The simplest and most compact mutual exclusion alg. Initially, flag[0]=flag[1]=false Prefix 0 flag[0]=true turn=1 while (flag[1] and turn=1) do{} CS 0 flag[0]=false suffix 0 Prefix 1 flag[1]=true turn=0 while (flag[0] and turn=0) do{} CS 1 flag[1]=false suffix 1

15 Solution for n processes Global Variable 1. Flag[0..n-1] – array of size n. 2. Turn. Initially, Turn=some no. between 0 and n-1 Idle if Pi is outside Csi Want-in if Pi wants to be in CSi in-CS if Pi is in CSi Flag[i]=

16 Solutions for n processes Pi Prefix i Repeat Flag[i]=want-in; j=Turn; while j  i do {if Flag[j]  idle then j=Turn else j=(j+1) mod n} Flag[i]=in-CS j=0 while (j<n) and (j=i or Flag[j]  in-CS) do {j=j+1} Until (j  n) and (Turn=i or Flag[Turn]=idle) Turn=i; CS i j=(Turn+1)mod n While (j  Turn) and (Flag[j]=idle) do{j=(i+1) mod n} Turn=j Flag[i]=idle

17 Three Environments 1. There is no central program to coordinate the processes. The processes communicate with each other through global variable. 2. Special hardware instructions 3. There is a central program to coordinate the processes.

18 Hardware Support Disable interrupt CS Enable interrupt Won’t work if we have multiprocessors

19 Special Machine Instructions 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 TS – Test and Set Boolean TS(i)= true if i=0; it will also set i to 1 false if i=1 Initially, lock=0 P i Prefix i While(¬ TS(lock)) do {} CS i Lock=0 suffix i It is possible that a process may starve if 2 processes enter the critical section arbitrarily often.

21 End of lecture 8 Thank you!