4.1.22. Lost update

  • T1
  • read_item(X);
  • X=X-N;


  • write_item(X);
  • read_item(Y);


  • Y=Y+N;
  • write_item(Y);
  • T2


  • read_item(X);
  • X=X+M;


  • write_item(X);
  • T1 X update overwritten