On Wed, 15 Nov 2023 22:33:03 GMT, Stuart Marks <sma...@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 (ยง12.6) for that object."_ > > src/java.base/share/classes/java/lang/ref/Reference.java line 495: > >> 493: * <a >> href="{@docRoot}/java.base/java/util/concurrent/package-summary.html#MemoryVisibility"><i>happen-before</i></a> >> 494: * the reference is removed from the queue by {@link >> ReferenceQueue#poll} >> 495: * or {@link ReferenceQueue#remove}. > > Should there be an explicit statement about an unsuccessful call having no > memory consistency effects? Perhaps something along the lines of "A failed {@code enqueue} has no defined memory consistency effects."? > src/java.base/share/classes/java/lang/ref/Reference.java line 505: > >> 503: * to return this reference even though the referent is still >> strongly >> 504: * reachable. That is, before the referent has reached the expected >> 505: * reachability level. > > The wording here is kind of awkward. We could try to wordsmith it, but there > is also the possibility that there are valid use cases for using enqueue(), > which makes this caution seem misplaced. An alternative would simply be to > omit this paragraph. Might be better to leave out the last sencence? > src/java.base/share/classes/java/lang/ref/Reference.java line 564: > >> 562: * {@code reachabilityFence(x)} >> 563: * <a >> href="{@docRoot}/java.base/java/util/concurrent/package-summary.html#MemoryVisibility"><i>happen-before</i></a> >> 564: * the garbage collector clears any reference to {code x}. > > This is a fairly low-level specification, so the relationship described here > is "reachabilityFence **hb** clearing". The relationships "clearing **hb** > enqueue" and "enqueue **hb** dequeue" are described elsewhere. Thus the > mention of Cleaner above seems misplaced. > > However, the whole chain of relationships > >> RF **hb** clearing **hb** enqueue **hb** dequeue **hb** cleaning-action > > is important. Should this be described somewhere? Perhaps in each of these places add a link to #Memory Consistency Properties ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/16644#discussion_r1400247244 PR Review Comment: https://git.openjdk.org/jdk/pull/16644#discussion_r1400251257 PR Review Comment: https://git.openjdk.org/jdk/pull/16644#discussion_r1400254380
