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.

Method used by P1	Method used by P2
while (S1 == S2); Critica1 Section S1 = S2;	while (S1 != S2); Critica1 Section S2 = not (S1);

Which one of the following statements describes the properties achieved?

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

Answer 1

The correct option is B Mutual exclusion but not progress

P1	P2
while (S1 == S2); Critica1 Section S1 = S2;	while (S1 != S2); Critica1 Section S2 = Not (S1);

We initially take $S_1=1,and{S}_2=1$
When P1 execute S1 = S2 then it do nothing and process P1 go into critical section after executing it make the value of S1 = S2 and when P1 execute then P2 not execute because that checks different conditon in while so both show mutual exclusion. But progress is not shown by these two because in this one process can be delayed infinitely.

Because again both P1 make its own condition true(S1 = S2 and S2 = not (S1);).