Process 1 opens file IN, "<in.txt"
Process 1 locks file IN
Process 1 opens file OUT, ">out.txt";
Process 1 locks file OUT
Process 2 opens file IN, "<in.txt";
Process 2 tries to lock IN and waits
  (The vulnerable window is greatly reduced if you use LOCK_NB, but not
  eliminated.  But then you have to close and reopen IN in a busy loop).
Process 1 reads the value "100" from IN
Process 1 writes the value "101" to OUT

In any order (or atomically):
  Process 1 closes IN (thereby unlocking it)
  Process 1 closes OUT (thereby unlocking it)
  Process 1 rename "out.txt" to "in.txt"

Process 2 acquires lock on file IN
Process 2 opens file OUT, ">out.txt";
Process 2 locks file OUT
Process 2 reads the value "100" from IN
Process 2 writes the value "101" to OUT

In any order (or atomically):
  Process 2 closes IN (thereby unlocking it)
  Process 2 closes OUT (thereby unlocking it)
  Process 2 rename "out.txt" to "in.txt"