RFR: 8277444: Race condition on Instrumentation.retransformClasses() and class linking [v3]

[Evgeny Astigeevich]

On Tue, 26 Aug 2025 13:39:14 GMT, Evgeny Astigeevich <eastigeevich at openjdk.org> wrote:

>> There is a race between `JvmtiClassFileReconstituter::copy_bytecodes` and `InstanceKlass::link_class_impl`.  `InstanceKlass::link_class_impl` can be rewriting bytecodes. `JvmtiClassFileReconstituter::copy_bytecodes` will not restore them to the original ones because the flag `rewritten` is `false`. This will result in invalid bytecode.
>> 
>> This PR adds a lock (`init_lock`) to the `copy_bytecodes` method to prevent reading bytecodes while they are being rewritten during class linking.
>> 
>> Tested fastdebug and release builds: Linux x86_64 and arm64
>> - The reproducer from JDK-8277444 passed.
>> - Tier1 - tier3 passed.
>
> Evgeny Astigeevich has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Symplify comments; Get JavaThread::current in variable

> Okay I ran the test case in the issue and I see why it wouldn't be reliable. I verified it with the new more minimalistic patch here: #26971

Thank you for the minimalistic patch.

I now have a version of the jtreg test which fails more reliably. 
The test always passes the the previous version of #26971.

BTW I have updated the title of [JDK-8277444](https://bugs.openjdk.org/browse/JDK-8277444).
The issue is the data race between copy_bytecodes and class linking. So we must guarantee no data race when copy_bytecodes is used.

I'll add the test to the PR soon.

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PR Comment: https://git.openjdk.org/jdk/pull/26863#issuecomment-3237447694
PR Comment: https://git.openjdk.org/jdk/pull/26863#issuecomment-3237450933