Question

Please take a look on the following pseudo-code:

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)); //RUN P0 and P1 parallel
}

I thought that a could implement a simple Mutual – Exclution solution using the code above. But it’s not working. Has anyone got an idea why?

Any help would really be appreciated!

Answer 1

Mutual Exclusion is in this exemple not guaranteed because of the following:

We begin with the following situation:

blocked = {false, false};
turn = 0;

P1 is now executes, and skips

  blocked[id] = false; // Not yet executed.

The situation is now:

blocked {false, true}
turn = 0;

Now P0 executes. It passes the second while loop, ready to execute the critical section. And when P1 executes, it sets turn to 1, and is also ready to execute the critical section.

Btw, this method was originally invented by Hyman. He sent it to Communications of the Acm in 1966