RFR: 8308387: CLD created and unloading list sharing _next node pointer leads to concurrent YC missing CLD roots [v6]

[Coleen Phillimore]

On Thu, 1 Jun 2023 08:24:25 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

>> [JDK-8307106](https://bugs.openjdk.org/browse/JDK-8307106) introduced the ability to walk the created CLDs list in the CLDG without a lock. This was primarily introduced to allow lockless concurrent CLD roots scanning for young collections in generational ZGC. However because the CLD _next node pointer is shared between the two list this can lead to a concurrent iteration of the created CLDs missing list entries. 
>> 
>> This change introduces a second _unloading_next node pointer which is used for the unloading CLDs list. The set_next is now maintains the invariant that it only ever unlinks is_unloading() CLDs and maintains a consistent view of the tail list for anyone reading the list concurrently.  
>> 
>> Testing: tier1-3 and tier1-7 with Generational ZGC
>
> Axel Boldt-Christmas has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Cleanup comments

Marked as reviewed by coleenp (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/14241#pullrequestreview-1456249669