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

Another thought, we need the fix in 11, 17 and 21, maybe in 8. As the current change is very small, it will be easy to backport it.

Additional enhancements can address 26 only.

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PR Comment: https://git.openjdk.org/jdk/pull/26863#issuecomment-3228563705