The problem is you do need some sort of atomic locking. There's nothing preventing both your processes from simultaneously obtaining "not locked" results from the file.

• Process A opens and reads the unlocked status
• Process A gets put to sleep
• Process B starts running
• Process B opens and reads the unlocked status
• B writes the locked status into the file and gets put to sleep
• A writes the locked status into the file and starts on its way
• B wakes up and hilarity ensues!