RFR: 8338379: Accesses to class init state should be properly synchronized [v2]

[Martin Doerr]

On Mon, 23 Sep 2024 07:17:50 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> See the bug for the discussion. We have not seen a clear evidence this is _the_ problem in the field, neither we were able to come up with a reproducer. We have found this gap by inspecting the code, while chasing a production bug.
>> 
>> In short, `InstanceKlass::_init_state` is used as the "witness" for initialized class state. When class initialization completes, it needs to publish the class state by writing `_init_state = _fully_initialized` with release semantics.
>> 
>> Various accessors that poll `IK::_init_state`, looking for class initialization to complete, need to read the field with acquire semantics. This is where the change fans out, touching VM, interpreter and compiler paths that e.g. implement clinit barriers. In some cases in assembler code, we can rely on hardware memory model to do what we need (i.e. acquire barriers/fences are nops).
>> 
>> I made the best _guess_ what ARM32, S390X, PPC64, RISC-V code should look like, based on what related code does for volatile loads. It would be good if port maintainers could sanity-check those.
>> 
>> Additional testing:
>>  - [x] Linux x86_64 server fastdebug, `all`
>>  - [x] Linux AArch64 server fastdebug, `all`
>>  - [x] GHA to test platform buildability + adhoc platform cross-compilation
>
> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Relax to just a release

Thanks for the explanation. This makes sense. Nevertheless, the aforementioned `membar_storestore()` follows the allocation immediately and it includes an acquire barrier for the current thread, too. So, the extra acquire is redundant. At least for the C1 code and probably at more places. This is not so obvious, so we may be able to live with what you have as long as performance is ok. Otherwise, we could still do a follow-up.

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PR Comment: https://git.openjdk.org/jdk/pull/21110#issuecomment-2379883568