[foreign-memaccess] [Rev 01] RFR: Simplify MemoryScope implementation to use StampedLock

[Paul Sandoz]

On Wed, 13 May 2020 20:38:27 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> I've been playing around a bit and discussed this offline with @JornVernee too. A realization I had some time ago is
>> that what we're trying to do here is a read/write lock, where multiple acquire can occur at the same time, but where
>> close of the root should be exclusive. I think all the machinery around `CLOSING` is essentially due to the code trying
>> to mimic that pattern.  This patch simplifies over the status quo, by using a `StampedLock` instead of managing races
>> manually through a `CLOSING` state. The tweaked implementation also adds back the atomicity w.r.t. close() vs. dup()
>> which was seeked in a previous attempt (see https://git.openjdk.java.net/panama-foreign/pull/160). Also, since now
>> there some guarantee that no acquire can take place while a close is also taking place (and pending acquires will be
>> invalidated - using the optimistic read logic), then we can just use a single long adder instead of two.  The state is
>> also simplified, since we just need a boolean flag.  I ran the `ParallelSum` benchmark and could not spot any obvious
>> regression compared to the previous code.
>
> Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix typos

Marked as reviewed by psandoz (Committer).

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PR: https://git.openjdk.java.net/panama-foreign/pull/163