GATE CSE 2010 | Question: 23

Question

Consider the methods used by processes $P1$ and $P2$ for accessing their critical sections whenever needed, as given below. The initial values of shared boolean variables $S1$ and $S2$ are randomly assigned.
$$\begin{array}{|l|l|}\hline \textbf{Method used by P1}  &  \textbf{Method used by P2}  \\ \hline  \text{while (S1 == S2);} & \text{while (S1 != S2);} \\  \text{Critical Section} & \text{Critical Section} \\  \text{S1 = S2;} & \text{S2 = not(S1);} \\\hline \end{array}$$
Which one of the following statements describes the properties achieved?

• A. Mutual exclusion but not progress
• B. Progress but not mutual exclusion
• C. Neither mutual exclusion nor progress
• D. Both mutual exclusion and progress

Comments

Is it an example of LIVELOCKed processes (where bounded waiting is satisfied but progress is not satisfied) ?

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But here it is says that  process P1 and P2 for accessing their critical section 'whenever needed' doesn't that means P1 or P2 can function concurrently

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@Reenakhanna yes you are right it is concurrent process but it is not progress due to while loop condition….

Answer 1

Here Mutual Exclusion is satisfied as here only one process can enter critical section at a particular time.

Suppose, $S1=1$ and $S2=0$ then $P2$ will be interested to enter critical section.But not $P1$.

Progress is not there as here $P2$ need to leave the critical section then only $P1$ can enter. But it might not happen so, condition of progress not satisfied.

$Ans: A$

Answer 2

To check mutual exclusion we need to check that atmost one process should enter the citical section and when one process is in the critical section then other process cant enter, Here we can see that S1 and S2 are mutually exclusive cases. Checking manually also when one process is in the CS then other cant enter. Hence Mutual exclusion is satisfied. 

To check progress we need to check if two or more processes want to enter the CS then atleast one process should enter, also if the process can also enter individually if they are interested without interference of other process.

In this question indvidual entering of process in the CS can be a problem, hence not satisfying Progress

So, Mutual exclusion but not progress