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

[Stuart Marks]

On Mon, 13 Nov 2023 22:31:16 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."_

src/java.base/share/classes/java/lang/ref/Reference.java line 401:

> 399:      * Avoid this race by ensuring the referent remains strongly-reachable until
> 400:      * after the call to clear(), using {@link #reachabilityFence(Object)} if necessary.
> 401:      *

The text about the potential race condition and the recommendation to use reachabilityFence() should be in an apiNote at the end of the method spec, after the following paragraph.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16644#discussion_r1394908730