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

[Brent Christian]

On Tue, 21 Nov 2023 22:58:09 GMT, Roger Riggs <rriggs 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 552:
> 
>> 550:      * of this method.
>> 551:      * Invocation of this method does not itself initiate reference processing,
>> 552:      * garbage collection, or finalization.
> 
> My understanding was that it is not the object instance that is being guarded, only that the reference holding the object is considered a strong root and is only used to delimit a range of bytecodes for which the reference is considered to be strong.
> In particular, the invocation of the method itself has no semantics, only that a control flow could reach that statement and the reference was considered strong as long as the reference was in a scope that included the reachability fence.

The existing docs refer to "invocation" of this method. I've continued with that, and in general have kept this at a bit higher level, in order to simplify understanding.

I'm open to more detailed wording, if it would improve understandability, (or if what I have is not functionally accurate).

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PR Review Comment: https://git.openjdk.org/jdk/pull/16644#discussion_r1411372474