+===============================+===============================+
| Process 1                     | Process 2                     |
+===============================+===============================+
| open the status file          |                               |  T
| read the status (status is 1) |                               |  i
+-------------------------------+-------------------------------+  m
|                               | open the status file          |  e
|                               | read the status (status is 1) |  |
|                               | write 2 to the status file    |  |
|                               | proceed                       |  |
+-------------------------------+-------------------------------+  v
| write 2 to the status file    |                               |
| proceed                       |                               |
+===============================+===============================+