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