x := 0

||

proc A {
  loop {
    if x = 0 then {
      [A enter critical section];
      [A exit critical section];
      x := 1
    }
  }
}

||

proc B {
  loop {
    if x = 1 then {
      [B enter critical section];
      [B exit critical section];
      x := 0
    }
    else [B wait]
  }
}