Question

P(sem) and V(sem) operations on semaphores are executed atomically. What will be the outcome of not executing P(sem) atomically? Specify where the interrupt occurs. Give one execution sequence of concurrent processes that will show a specific Condition (Mutual Exclusion or Starvation or Progress Condition) violated. Be detailed in your explanation.

Answer 1

A wait(P(sem)) operation atomically decrements the value associated with a semaphore. If two wait operations are executed on a semaphore when its value is 1, if the two operations are not performed atomically, then it is possible that both operations might proceed to decrement the semaphore value, thereby violating mutual exclusion.

In following code there are two processes 0 and 1. They take turns executing their critical sections. Thus mutual exculusion is present

Process 0 while (true) { while (turn != 0); /* wait until own turn */ <<< critical section >>> turn = 1; //After critical section, providing turn to other process <<< code outside critical section >>> }

Process 1
while (true)

{

while (turn != 1); /* wait until own turn */

<<< critical section >>>

turn = 10; //After critical section, providing turn to other process

<<< code outside critical section >>>

}

Consider P0, a process that must be in the critical section 70% of the time, and another process, P1, that must be in the critical section 1% of the time. P1's infrequent use of the critical section will block P0 most of the time (Progress criteria violated)