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

[Aleksey Shipilev]

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

All right, granted. We can make an argument that a release store to `IK::init_state` can be matched with cumulative barrier like `storestore` at the end of C1-compiled allocation code. That said, it looks quite fragile, since: a) it depends on cumulative properties of low-level hardware primitives, and b) it likely only holds true for C1, as C2 normally coalesces header-protecting `storestore` with the final field `storestore`.

Given that we expect no perf problems on this seemingly rare path, I prefer not to go into exploiting those specifics, unless you feel strongly otherwise :)

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PR Comment: https://git.openjdk.org/jdk/pull/21110#issuecomment-2380238637