\$s
+=====================================================+
|                         CPU                         |
+--------------------------+--------------------------+
|         thread 1         |         thread 2         |
+==========================+==========================+
| - ...                    |                          |
| - read $s's refcount     |                          |
|   - $s is locked         |                          |
|   - refcount is read     |                          |
|   - $s is unlocked       |                          |
| - add one                |                          |
+--------------------------+--------------------------+
|                          | - ...                    |
|                          | - read $s's refcount     |
|                          |   - $s is locked         |
|                          |   - refcount is read     |
|                          |   - $s is unlocked       |
|                          | - add one                |
|                          | - write $s's refcount    |
|                          |   - $s is locked         |
|                          |   - write $s's refcount  |
|                          |   - $s is unlocked       |
|                          | - ...                    |
+--------------------------+--------------------------+
| - write $s's refcount    |                          |
|   - $s is locked         |                          |
|   - write $s's refcount  |                          |
|   - $s is unlocked       |                          |
| - ...                    |                          |
+==========================+==========================+
                => refcount is wrong <=