process synchronization

Question

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

Answer 1

the correct answer is Option B

Comments

initially both flag[i] and flag[j] are initialized to false

say that process P_i wants to enter into critical section

then it will make flag[i] = true(saying that it wants to enter the critical section) and it will first give the turn to j (turn = j)

then in the while loop it will check for the condition 

while(flag[j] && turn == j)

now flag[j] is false the while loop will break and process P_i will enter into the critical section

thus progress is guaranteed here as the process which is not interested (P_j) to enter the critical section does not stop the other process (P_i) which is interested to enter into the critical section

while P_i is in critical section what if process P_j also wants to enter critical section

then it will make flag[j] = true and turn = i

then it will check for the condition while(flag[i] && turn == i)

in our case it is true and process P_j will be in the while loop do nothing till process P_i gets out of critical section

thus only one process can enter into the critical section at a time so mutual exclusion is guaranteed

since Pi does not change the value of turn while executing the while statement Pi will enter critical section after at most one entry by Pj thus bounded waiting is also guaranteed

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@Tamil

1.flag[i]=true;

2.turn=j;

3.while(P);

if process Pj wants to enter into critical section after line 1 then both flag[i] and flag [j] are true and turn is set to j.In this case Pj will blocked .And again for Pi turn=i ,it will enter into critical section .By this Pj will never go into critical section .And according to peterson's the process which sets turn value first will enter into critical section first .

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if process Pj wants to enter into critical section after line 1 then both flag[i] and flag [j] are true and turn is set to j.

when Pj executes the second line it will give turn to i not to j

to explain yours

P_i will execute first line and set flag[i] = true and gets preempted

P_j will execute first and second line and set flag[j] = true and turn = i and will check for the condition in the while loop while(flag[i] && turn == i) and it will get stuck in the while loop, after some time it will get preempted

then again P_i will execute line two and three  turn = j and this will also stuck in the while loop till it get preempted

then again when P_j gets its chance it will check the while loop while(flag[i] && turn == i)

but now turn = j so the while loop will break and P_j which access the turn variable first will enter the critical section