RFR: 8361952: Installation of MethodData::extra_data_lock() misses synchronization on reader side

[Thomas Schatzl]

On Fri, 11 Jul 2025 12:23:18 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Hi all,
>> 
>>   please review this change that fixes some recently introduced atomic installation of a mutex, where the memory barrier (`load_acquire`) on the reader side. Without it the reader might get a valid pointer to the `Mutex` created on the fly, without it being initialized properly.
>> 
>> Found during code inspection for https://bugs.openjdk.org/browse/JDK-8361706 ; due to some suspicious hangs in the `MutexLocker` while cleaning klasses during class unloading in parallel (multiple threads hanging in `MethodData::clean_method_data`), executing the `vmTestbase/nsk/jvmti/RedefineClasses/StressRedefine/TestDescription.java` test.
>> 
>> Testing: gha
>> 
>> Thanks,
>>   Thomas
>
> OK, sure.

Thanks @shipilev @coleenp @dholmes-ora for your reviews

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PR Comment: https://git.openjdk.org/jdk/pull/26262#issuecomment-3068518246