Question

The following code fragment is a 2-process "solution" to the critical section problem 

try [i] = true ; 

while (incs[j]) no-op; 

while (turn=j and try[j]) no-op; 

incs [i] = true ; 

critical section 

try [i] = false; 

incs [i] = false; 

turn = j ; 

Does the above "solution" meet all 3 conditions of critical sections? Please explain your answer.

Answer 1

Yes,

For process table:

repeat

       ………………….

         critical section

       ……………………

      remainder section

until FALSE

We have, Solution to the Critical Section Problem must meet three conditions defined below:

1. Mutual exclusion: If a process is executing in its critical section, no other process is executing in its critical section
2. Progress: If no process is executing in its critical section and there exists some processes that wish to enter their critical sections, then only those processes that are not executing in their remainder section can participate in the decision of which will enter its critical section next, and this decision cannot be postponed indefinitely
   • If no process is in critical section, can decide quickly who enters
   • Only one process can enter the critical section so in practice, others are put on the queue
3. Bounded waiting: There must exist a bound on the number of times that other processes are allowed to enter their critical sections after a process has made a request to enter its critical section and before that request is granted
   • The wait is the time from when a process makes a request to enter its critical section until that request is granted
   • In practice, once a process enters its critical section, it does not get another turn until a waiting process gets a turn (managed as a queue)

In ques,

In second line,

While (incs[j]) no-op :It is saying that while process ‘I’ is in critical section, no other process is allowed to enter into it. That is meeting out first condition of Mutual exclusion.

Lines,

Try[i]=False

Incs[i]=False

Turn j

It is an example of progress, i.e. Process ‘j’ was waiting earlier when process ‘I’ was in critical section ,now when ‘I ‘ is not in critical section ,waiting process ‘j’ is allowed to enter into the critical section.

Also ,it is na example of Bounded waiting i.e. once a process enters its critical section, it does not get another turn until a waiting process gets a turn (managed as a queue)

Thanks !