RFR: 8324881: ObjectSynchronizer::inflate(Thread* current...) is invoked for non-current thread [v5]

[David Holmes]

On Thu, 1 Feb 2024 12:40:26 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

>> The `ObjectSynchronizer` has always assumed that the `current` parameters are both the current thread as well as the thread that is doing the locking. The only time that we are entering on behalf of another thread is when doing re-locking in deoptimization. This has worked because the deoptee thread is suspended. However ResourceMarks have been using the wrong thread when logging is enabled.
>> 
>> This change `ObjectSynchronizer` instruments the relevant methods with both a `JavaThread* locking_thread` as well as `[Java]Thread* current` to be able to use the correct thread for ResourceMarks. 
>> 
>> Having the `inflate` care about a `locking_thread` is a little unpleasant in my opinion. But it is required for LM_LIGHTWEIGHT. 
>> Would probably be cleaner if the inflate for LM_LIGHTWEIGHT was it's own thing, as it does not share the whole INFLATING protocol. But seems like a future RFE to refactor this code.
>> 
>> Can reproduce a crash by modifying `test/jdk/com/sun/jdi/EATests.java` and using `-XX:DiagnoseSyncOnValueBasedClasses=2` with LM_LEGACY or running `test/jdk/com/sun/jdi/EATests.java` with LM_LIGHTWEIGHT/LM_MONITOR and `-Xlog:monitorinflation=trace`. 
>> 
>> Could extend this test to capture this regression in the future (or creating a new test based on the same infrastructure). Will give it an attempt, so we have a regression test for this. But these tests get rather involved as the require a lot of jvmti setup.
>
> Axel Boldt-Christmas has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix enter_for LockingMode == LM_LEGACY and strengthen assert

Thanks for that additional separation and refactoring. I still have a general query below.

src/hotspot/share/runtime/synchronizer.cpp line 1372:

> 1370:       if (LockingMode == LM_LIGHTWEIGHT && inf->is_owner_anonymous() &&
> 1371:           thread != nullptr && thread->lock_stack().contains(object)) {
> 1372:         inf->set_owner_from_anonymous(thread);

I thought one of the invariants of lightweight locking was that a transition from anonymously owned to actual owner, can only be performed by the actual owner?

That said, can we actually reach here if not with the current thread? If we are re-locking for deopt then, IIUC, the object has not yet escaped, and has to be unlocked and not with an inflated monitor. In such a case we can't be racing with hashcode inflation either. But perhaps there are other circumstances where re-locking is needed?

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PR Review: https://git.openjdk.org/jdk/pull/17626#pullrequestreview-1858288427
PR Review Comment: https://git.openjdk.org/jdk/pull/17626#discussion_r1475529283