RFR: 8314480: Memory ordering spec updates in java.lang.ref

[Brent Christian]

On Wed, 15 Nov 2023 22:45:46 GMT, Stuart Marks <smarks at openjdk.org> wrote:

>> Classes in the `java.lang.ref` package would benefit from an update to bring the spec in line with how the VM already behaves. The changes would focus on _happens-before_ edges at some key points during reference processing.
>> 
>> A couple key things we want to be able to say are:
>> - `Reference.reachabilityFence(x)` _happens-before_ reference processing occurs for 'x'.
>> - `Cleaner.register()` _happens-before_ the Cleaner thread runs the registered cleaning action.
>> 
>> This will bring Cleaner in line (or close) with the memory visibility guarantees made for finalizers in [JLS 17.4.5](https://docs.oracle.com/javase/specs/jls/se18/html/jls-17.html#jls-17.4.5):
>> _"There is a happens-before edge from the end of a constructor of an object to the start of a finalizer (§12.6) for that object."_
>
> src/java.base/share/classes/java/lang/ref/ReferenceQueue.java line 187:
> 
>> 185:      * {@link Reference#enqueue()} instead of by the garbage collector, its
>> 186:      * former referent (which has since been cleared) could still be strongly
>> 187:      * reachable.
> 
> See my comments on the corresponding Reference.enqueue() apiNote. I'm not sure we want to have this issue sprinkled around in the apiNotes for several methods, including others here on ReferenceQueue.
> 
> If we do want to address this issue -- and it's not clear to me that we do -- then perhaps we should do it in a central location?

We can cover this issue just in `Reference.enqueue()`. I'll take out these `@apiNote`s, and put in `@see Reference.enqueue()`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16644#discussion_r1411395204