RFR: 8314480: Memory ordering spec updates in java.lang.ref [v29]

[Alan Bateman]

On Mon, 29 Apr 2024 21:17:24 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:
> 
>   small grammar fixes

src/java.base/share/classes/java/lang/ref/ReferenceQueue.java line 39:

> 37:  *
> 38:  * <p><a href="{@docRoot}/java.base/java/lang/ref/package-summary.html#MemoryConsistency">Memory consistency effects</a>:
> 39:  * The enqueueing of a reference on a queue (by the garbage collector, or by a

In passing, I wonder if you're actually meant to say "on a queue" here.  Elsewhere the API docs speak of adding and removing, e.g. the enqueue method speaks of adding a reference "to" the queue.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16644#discussion_r1593659540