RFR: 8338379: Accesses to class init state should be properly synchronized
Aleksey Shipilev
shade at openjdk.org
Sat Sep 21 06:07:06 UTC 2024
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. Current patch makes a seqcst write, which is stronger than strictly necessary. I think it is okay to be extra paranoid on rarely-executed class initialization path.
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
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Commit messages:
- Initial version
Changes: https://git.openjdk.org/jdk/pull/21110/files
Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21110&range=00
Issue: https://bugs.openjdk.org/browse/JDK-8338379
Stats: 27 lines in 17 files changed: 12 ins; 0 del; 15 mod
Patch: https://git.openjdk.org/jdk/pull/21110.diff
Fetch: git fetch https://git.openjdk.org/jdk.git pull/21110/head:pull/21110
PR: https://git.openjdk.org/jdk/pull/21110
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