GATE IT 2007 | Question: 10

Question

Processes $P1$ and $P2$ use critical_flag in the following routine to achieve mutual exclusion. Assume that critical_flag is initialized to FALSE in the main program.

get_exclusive_access ( )
{
    if (critical _flag == FALSE) {
        critical_flag = TRUE ;
        critical_region () ;
        critical_flag = FALSE;
    }
}

Consider the following statements.

1. It is possible for both $P1$ and $P2$ to access critical_region concurrently.
2. This may lead to a deadlock.

Which of the following holds?

• A. (i) is false (ii) is true
• B. Both (i) and (ii) are false
• C. (i) is true (ii) is false
• D. Both (i) and (ii) are true

Comments

What about bounded waiting and progress
Bounded waiting is not gaureented
And progress always gaureented

Plz correct me

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For Deadlock the necessary condition is Mutual Exclusion..isn't it?

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Progress isn’t guaranteed because P2 will depend on P1 for critical_flag = FALSE if only enters the IF block. P1 will perform critical_flag = TRUE, access critical region, and then P2 will be blocked again at its IF block as critical_flag = TRUE. Thus, P2 will be depending on P1 (which is no more inside the critical region) for its further execution. Correct me if I am wrong.

Answer 1

(C) Both process can run the critical section concorrently. Lets say $p_{1}$ starts and it enters inside if claus and just after its entertence and before execution of critical_flag = TRUE, a context switch happens and $p_{2}$ also gets entrance since the flag is still false. So, now both process are in critical section! So, (i) is true. (ii) is false there is no way that flag is true and no process' are inside the if clause, if someone enters the critical section, it will definetly make flag = false. So. no. deadlock.

Comments

after executing line 2, p1 got blocked and never returned?

where that will got blocked? Why that will never returned?

Blocking of a process cannot happen unnecessarily

right? 

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Yes probably, just had this in mind.

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If no Mutual Exclusion is the case how can deadlock happen in the first place as Mutual Exclusion is one of the four pillars of deadlock and also that pillar that is the strongest and cannot be violated at any cost? So no deadlock in any case here. This result is followed by the first statement being true.

Answer 2

Say P1 starts first and executes statement 1, after that system context switches to P2 (before executing statement 2), and it enters inside if statement, since the flag is still false.
So now both processes are in critical section!! so (i) is true.. (ii) is false
By no way it happens that flag is true and no process’ are inside the if clause, if someone enters the critical section, it will definitely make flag = false. So no deadlock.

Answer 3

here deadlock won't occur as the condition here is "if(critical_flag==false)"  not while(critical_flag==false);.

consider process P1 currently in critical section  preempts before setting critical _flag=False.

process P2 performs condition (True==false) gives false and hence comes out of the if condition and hence its execution is completed.there is no loop here (unlike while();)and hence no dependency on other processes.

Hence no deadlock.

for first answer both can enter critical sections as preemption could be done after if condition and before setting critical_flag as true  resulting multiple processes accessing critical sections at same time.