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Consider Peterson's algorithm for mutual exclusion between two concurrent processes i and j. The program executed by process is shown below.

repeat
    flag[i] = true;
    turn = j;
    while (P) do no-op;
    Enter critical section, perform actions, then
    exit critical section
    Flag[i] = false;
    Perform other non-critical section actions.
Until false;

For the program to guarantee mutual exclusion, the predicate P in the while loop should be

  1. flag[j] = true and turn = i
  2. flag[j] = true and turn = j
  3. flag[i] = true and turn = j
  4. flag[i] = true and turn = i
asked in Operating System by Veteran (68.8k points)
edited by | 2.1k views

I think if we will change 

turn = j; --> turn = i; 

Then also it will work. Please correct me if i am wrong.

yes it will work but condition P statement will change to flag[j]==true and turn==i
option B

5 Answers

+25 votes
Best answer
ans is B. suppose two processes p1 and p2. to gurantee mutual exclusion only 1 process must be in CS at a time. now, shared variable is turn P1 P2 f[1]=true f[2]=true turn=2 p1 will now check for condition while(f[2]=true and turn=2) then it will go for busy wait... then p2 will execute and it will update turn=1 p2 will now check condition while(f[1]=true and turn=1) then it will go for busy wait, but here as the value of turn is updated by p2; p1 will be able to enter into CS as the condition will become false therefore p1 will enter CS and p2 will be in busy wai until the p1 will come out and make f[1]=false hence, no two process can enter into CS at a time so predicate p is option B.
answered by Boss (8.4k points)
selected by
C is also correct ans...........

@ jayendra what about option c can you explain....

@bikram sir Are B and C both the correct answer?
Here the question should mention that this is the code executed by the process Pi.Otherwise it creates  confusion.

It is a tricky question.We should eliminate options if possible:-

c:) creates deadlock

a& d:)will fail mutual inclusion as the process has set turn =j. Now it will check turn =i in for loop .so,both can enter.So remaining is b.which is surely the answer

Let us say both the processes tries to enter now both set flag to 1 but the key is turn variable.it will have i or j ,so one of the process will fail and other will pass and enter.
+5 votes

If process Pi wants to enter into CS then it will make Flag[i]=True and turn=j. Now for while loop, consider 2 points:

(i)  If we take turn=i, then it already makes condition False in while loop and thus both processes can enter simultaneously into  CS. So, take turn=j

(ii)  if we take flag[i]=TRUE, then the condition will become like process ' i ' wants to enter which made flag[i]=TRUE and        turn=j already, so it will always remain in while loop forever. So, take Flag[j]=TRUE

Thus (B) is the answer for sure :)

answered by Active (1.1k points)
+2 votes

Option C is correct(Best option)

First of all its not a complete implementation of Peterson Algorithm. Here we need to check Mutual Exclusion. Now for that Process i is in CS and Process j is trying to enter into CS. How I know it ? It can be seen in code " turn = j " that means Process j is trying to enter CS. Therefore to make sure Mutual Exclusion is there Process j should keep on iterating in the while( ) loop while Process i is in CS . Now since Process i is in CS we need to make sure that when Process i exits CS then only Process j enters. Therefore for that to happen Process i must execute " Flag[i] = false". From this we can take condition that  "Flag[i] = true " should be put in while loop. Secondly since "turn = j" therefore " turn == j " should also be added in while loop. Since turn = j is given therefore i would best choose option c rather d.

answered by Junior (627 points)
edited by
Here first flag[i] and turn vaiables are set and then compared with same value:

flag[i] = true; 
turn = j; 
while(flag[i] == true and turn ==j); // it will always give infinite loop

 

but when the second process executes line no.2 won't the condition in while of the first process become(true&&false) and it would enter the critical section?
–2 votes

Ans should be (c) as it should be FLAG[other] which is i and turn of current is checked i.e j

answered by (383 points)
–4 votes
ans is C is correct because in case of option B, the process will enter simultaneously enter into the critical section but incase of C mutual exclusion will be ensured.
answered by (365 points)


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