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));

}

This software solution to the mutual exclusion problem for two processes is proposed in [HYMA66]. Find a counterexample that demonstrates that this solution is incorrect. It is interesting to note that even the Communications of the ACM was fooled on this one.