Question

Consider the following program:
boolean blocked [2];
int turn;
void P (int id)
{
while (true) {
blocked[id] = true;
while (turn != id) {
while (blocked[1-id])
/* do nothing */;
turn = id;
}
/* critical section */
blocked[id] = false;
/* remainder */
}
}
void main()
{
blocked[0] = false;
blocked[1] = false;
turn = 0;
parbegin (P(0), P(1));
}

Answer 1

Consider the case in which turn equals 0 and P(1) sets blocked[1] to true and then finds blocked[0] set to false. P(0) will then set blocked[0] to true, find turn = 0, and enter its critical section. P(1) will then assign 1 to turn and will also enter its critical section. The error was pointed out in [RAYN86].

 P(1) will then assign 1 to turn and will also enter its critical section. The error was pointed out in [RAYN86].