In the proposal, we are talking about adding extra security for the
shared window, not something to replace RMW. As you read, lock/unlock
is allowed only in the local process.
>So unless I'm missing something painfully obvious, I don't understand
>how this can be used to implement general RMW operations...
I think I wanted to say the opposite.
A lock mechanism can be implemented at the user level using RMW
without help of the lock/unlock functions. In this sense, they may
seem superfluous. However, the direct lock/unlock interface provides a
more intuitive way for the end user to protect her window while she
is doing some work in the window area. It is also likely to have
little impact on performance because the operation is local.