CS 3204: Synchronization Example

Problem description

3 types of processes: A, B, C
Critical Resource: T
Criteria:
- ME B & C
- ME A & C
- ME among the As
- ME among the Cs
- Cs have priority over As and Bs

Monitor Solution

ABC: Monitor
var Aok, Bok, Cok: Condition;
    WC, WB, RB: Integer <-- 0;
    ABusy, CBusy: Boolean <-- FALSE;

ReqA

if (WC > 0 || ABusy || CBusy)
  Aok.wait; 
ABusy = TRUE;

RelA

ABusy = FALSE;
if (WC == 0)
  Aok.signal
else if (RB == 0)
        Cok.signal;

ReqB

if (WC > 0 || CBusy) {
  WB++;
  Bok.wait;
  WB--;
}
RB++;

RelB

RB--;
if (RB == 0 && !ABusy) {
  Cok.signal;
}

ReqC

if (RB > 0 || CBusy || ABusy) {
  WC++;
  Cok.wait;
  WC--;
}
CBusy = TRUE;

RelC

CBusy = FALSE;
if (WC > 0) {
  Cok.signal;
else {
  Aok.signal;
  while (WB > 0)
     Bok.signal;
}

Semaphore Solution

var MEAC, MEBC, MEC: Semaphore <-- 1;
    semWC, semWB, stopA, stopB: Semaphore <-- 1;
    WC, WB: integer <-- 0;

Process A

P(stopA); // For C's priority
V(stopA);
P(MEAC); // For ME between A C
<CS>
V(MEAC);

Process B

P(stopB); // For C's priority
V(stopB);
P(semWB);
WB++;
if (WB == 1)
  P(MEBC); // For ME between BC
V(semWB);
<CS>
P(semWB)
WB--;
if(WB == 0)
  V(MEBC);
V(semWB);

Process C

P(semWC); // for updating WC
WC++;
if (WC == 1) {
  P(stopB); // C's priority over B
  P(stopA); // C's priority over A
  P(MEBC); // ME between C B
  P(MEAC); // ME between A C
}
V(semWC);
P(MEC);
<CS>
V(MEC);
P(semWC);
WC--;
if (WC == 0) {
  V(stopA);
  V(stopB);
  V(MEAC);
  V(MEBC);
}
V(semWC);