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

[Brent Christian]

On Tue, 21 Nov 2023 09:08:06 GMT, Viktor Klang <vklang 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/Reference.java line 405:
> 
>> 403:      * <p> This method is invoked only by Java code; when the garbage collector
>> 404:      * clears references it does so directly, without invoking this method. The
>> 405:      * {@link #enqueue} method also clears references directly, without invoking
> 
> The last sentence might make sense to have as an apiNote? 🤔

I will remove that sentence here, and instead update the `enqueue()` doc to say that the GC clears and enqueues references without calling `clear()` or `enqueue()`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16644#discussion_r1408644358