i+=1

         CPU
----------+----------
 thread 1 | thread 2
==========+==========
 ...      |             T
 load i   |             i
 add  1   |             m
----------+----------   e
          | ...         |
          | load i      |
          | add  1      v
          | save i
          | ...
----------+----------
 save i   |
 ...      |