On Wed, 31 Jan 2024 01:13:18 GMT, Brent Christian <bchri...@openjdk.org> wrote:
>> src/java.base/share/classes/java/lang/ref/package-info.java line 109: >> >>> 107: * >>> 108: * <li>The clearing of a reference by the garbage collector >>> <i>happens-before</i> >>> 109: * the garbage collector enqueues the reference.</li> >> >> Is it worth specifying this middle step? Is there a way to tell that >> something has been enqueued without removing the reference or calling the >> deprecated (and very dubious) isEnqueued method? Could we just remove this >> paragraph, and instead start the next one with "The clearing of a reference >> by the garbage collector ..." > > Here, we are building a chain of _happens-befores_ that reaches from RF to > dequeue (and on, to the running of the cleaning action, in the case of > Cleaner). > > A ref can also be cleared by a call to clear(), but that has no memory > consistency effects. > > So I think it's worth clarifying here that ref-clearing only forms a "link" > in the _happens-before_ chain **_when performed by the GC_**. I had also wondered why we don't just have a single edge GC-clear _happens-before_ dequeue. The reason is that there is a potential gap between GC-clear and GC-enqueue, during which some application thread can call the `enqueue` method. If the application thread's call successfully enqueues the ref, the HB edge is between the application thread's prior actions and the dequeue, and this **breaks** the HB chain that started at `reachabilityFence` and that would follow GC's actions. Thus, to keep the chain intact, GC must clear the ref and also successfully enqueue the ref. This is rather more complex than I would have liked. It would be nicer if clear-and-enqueue were atomic (either performed by GC, or performed by an application thread's call to `enqueue`, and not a mixture). However, this isn't supported by the specification, and the implementations I'm aware of don't do this. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/16644#discussion_r1527092160
