$s = $s + 1
+=====================================================+
|                         CPU                         |
+--------------------------+--------------------------+
|         thread 1         |         thread 2         |
+==========================+==========================+
| - ...                    |                          |
| - copy $s into temp      |                          |
|   - $s is locked         |                          |
|   - copy $s into temp    |                          |
|   - $s is unlocked       |                          |
| - add one to temp        |                          |
+--------------------------+--------------------------+
|                          | - ...                    |
|                          | - copy $s into temp      |
|                          |   - $s is locked         |
|                          |   - copy $s into temp    |
|                          |   - $s is unlocked       |
|                          | - add one to temp        |
|                          | - assign temp to $s      |
|                          |   - $s is locked         |
|                          |   - assign temp to $s    |
|                          |   - $s is unlocked       |
|                          | - ...                    |
+--------------------------+--------------------------+
| - assign temp to $s      |                          |
|   - $s is locked         |                          |
|   - assign temp to $s    |                          |
|   - $s is unlocked       |                          |
| - ...                    |                          |
+==========================+==========================+
                   => $s is wrong <=