RFR: 8343394: Make MemorySessionImpl.state a stable field [v4]

[Per Minborg]

On Fri, 1 Nov 2024 02:44:15 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> Hi,
>> 
>> This patch makes `MemorySessionImpl.state` a `Stable` field so that liveness check of non-closeable scopes such as the global scope can be elided.
>> 
>> Currently, the `state` field is overloaded with 2 responsibilities, to act as a communication device between `close` and `checkValidState`, as well as a communication device between `close`, `acquire`, and `release`. This patch separates those concerns into `state` and `acquireCount`, allowing `state` to be marked as `@Stable`.
>> 
>> With the patch, in `MemorySegmentGetUnsafe`, `panama` is able to be on par with `unsafe`:
>> 
>>     Benchmark                      Mode  Cnt  Score   Error  Units
>>     MemorySegmentGetUnsafe.panama  avgt   30  0.340 ± 0.008  ns/op
>>     MemorySegmentGetUnsafe.unsafe  avgt   30  0.332 ± 0.004  ns/op
>> 
>> For reference this is the results without this patch:
>> 
>>     Benchmark                      Mode  Cnt  Score   Error  Units
>>     MemorySegmentGetUnsafe.panama  avgt   30  0.420 ± 0.019  ns/op
>>     MemorySegmentGetUnsafe.unsafe  avgt   30  0.329 ± 0.003  ns/op
>> 
>> Please kindly review, thanks very much.
>
> Quan Anh Mai has updated the pull request incrementally with one additional commit since the last revision:
> 
>   wait for the close operation to complete on acquire failures

src/java.base/share/classes/jdk/internal/foreign/SharedSession.java line 90:

> 88:         }
> 89: 
> 90:         STATE.setOpaque(this, CLOSED);

Why are we using opaque semantics here and not volatile?

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PR Review Comment: https://git.openjdk.org/jdk/pull/21810#discussion_r1825600258