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));
}
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.
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