Race Condition

Description:

• Nondeterministic behavior caused by the times at which two or more threads access a shared variable
• Debug race condition is hard
• example: Bounded-buffer Problem, Producer and Consumer Problem
  • Writer Process P, Producer

    while (true) {
    	// Busy-wait loop: Wait if buffer is full
    	while (counter == SIZE);
    	// Insert nextItem into the buffer at position 'in'
    	buf[in] = nextItem;
    	// next position
    	in = (in + 1) % SIZE;
    	counter++;
    }

  • Reader process Q: Consumer

    while (true) {
    	// Busy-wait loop: Wait if the buffer is empty
    	while (counter == SIZE);
    	// Read
    	nextItem = buf[out];
    	// next position
    	out = (out+1) % SIZE;
    	counter --;
    }

  • Where counter is shared variable
    • Implementation of counter ++
      • register1 = counter
      • register1 = register1 + 1
      • counter = register1
    • Implementation of counter—
      • register2 = counter
      • register2 = register2 - 1
      • counter = register2
  • Process P and Q can get different counter values at the same time if line 3 of both implementations are executing