RFR: 8314480: Memory ordering spec updates in java.lang.ref [v6]

[Kim Barrett]

On Wed, 10 Jan 2024 22:12:40 GMT, Brent Christian <bchristi 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."_
>
> Brent Christian has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Tweak Reference.enqueue memory consistency effects wording

src/java.base/share/classes/java/lang/ref/Reference.java line 508:

> 506:      * Use of this method allows the registered queue's
> 507:      * {@link ReferenceQueue#poll} and {@link ReferenceQueue#remove} methods
> 508:      * to return this reference even though the referent is still strongly

Perhaps s/is still/may still be/

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PR Review Comment: https://git.openjdk.org/jdk/pull/16644#discussion_r1463145471