Critical Section

Code that access a shared variable

Must not be concurrently executed by more than one thread

Example

int total = 0;
void add() {
  for (int i=0; i<N; i++) {
    total++;
  }
}
void sub() {
  for (int i=0; i<N; i++) {
    total--;
  }
}

If one thread executes add and one executes sub, we won’t know the value for total

Why does this happen

/* Thread 1 */
void add() {
  for (int i=0; i<N; i++) {
    lw r9, 0(r8) /* total++ */
    addi r9, 1
    sw r9, 0(r8)
  }
}

/* Thread 2 */
void sub() {
  for (int i=0; i<N; i++) {
    lw r9, 0(r8) /* total++ */
    subi r9, 1
    sw r9, 0(r8)
  }
}

At the assembly level, these instructions are actually 3 different instructions