A concurrent system consists of 3 processes using a shared resource R in a non-preemptible and mutually exclusive manner...

Question

A concurrent system consists of 3 processes using a shared resource R in a non-preemptible and mutually exclusive manner. The processes have unique priorities in the range 1...3, 3 being the highest priority. It is required to synchronize the processes such that the resource is always allocated to the highest priority requester.

Shared data:
  mutex semaphore = 1  /* initialized to 1 */
  process[i] semaphore = 0  /* initialized to 0 */
  R_requested[i] boolean = false  /* initialized to false */
  busy boolean = false  /* initialized to false */

Code for processes:
  begin process
    my_priority_integer;
    my_priority := /* in the range 1..3 */
    repeat
      request_R(my_priority);
      /* use shared resource R */
      release_R(my_priority);
    forever
  end process

Procedures:
  procedure request_R(priority);
  begin
    P(mutex);
    if busy = true then
      begin
        R_requested[priority] := true;
        V(mutex);
        P(process[priority]);
      end
    else
      begin
        busy := true;
        V(mutex);
      end
  end;

procedure release_R(priority);
  begin
    /* TO BE COMPLETED */
  end;

Give the pseudo-code for the procedure release_R.

A. Set busy=false and signal the highest priority waiting process
B. Simply call V(mutex) to release mutual exclusion
C. Set R_requested[priority]=false and V(process[priority])
D. Busy-wait until no process is requesting R, then set busy=false

Accepted Answer

release_R scans R_requested[] from priority 3 to 1; if a waiter exists it clears the flag and signals that process's semaphore; otherwise it sets busy=false. This always grants R to the highest-priority requester. Three processes share a resource R and each has a unique priority (1, 2, or 3, where 3 is highest). When a process is done with R, it must give R to whichever waiting process has the highest priority. The request_R procedure uses semaphores and a boolean array to track who is waiting. We need to write release_R that correctly hands over R to the highest-priority waiter.