function mymodule_long_operation() {
  if (lock_acquire('mymodule_long_operation')) {
    // Do the long operation here.
    // ...
    lock_release('mymodule_long_operation');
  }
}

function mymodule_long_operation() {
  if (lock_acquire('mymodule_long_operation')) {
    // SOMETHING GOES WRONG HERE!
    lock_release('mymodule_long_operation');
  }
}