RFR: 8313816: Accessing jmethodID might lead to spurious crashes [v11]

[David Holmes]

On Wed, 29 Nov 2023 11:49:31 GMT, Jaroslav Bachorik <jbachorik at openjdk.org> wrote:

>> Please, review this fix for a corner case handling of `jmethodID` values.
>> 
>> The issue is related to the interplay between `jmethodID` values and method redefinitions. Each `jmethodID` value is effectively a pointer to a `Method` instance. Once that method gets redefined, the `jmethodID` is updated to point to the last `Method` version. 
>> Unless the method is still on stack/running, in which case the original `jmethodID` will be redirected to the latest `Method` version and at the same time the 'previous' `Method` version will receive a new `jmethodID` pointing to that previous version.
>> 
>> If we happen to capture stacktrace via `GetStackTrace` or `GetAllStackTraces` JVMTI calls while this previous `Method` version is still on stack we will have the corresponding frame identified by a `jmethodID` pointing to that version.
>> However, sooner or later the 'previous' class version becomes eligible for cleanup at what time all contained `Method` instances. The cleanup process will not perform the `jmethodID` pointer maintenance and we will end up with pointers to deallocated memory. 
>> This is caused by the fact that the `jmethodID` lifecycle is bound to `ClassLoaderData` instance and all relevant `jmethodID`s will get batch-updated when the class loader is being released and all its classes are getting unloaded. 
>> 
>> This means that we need to make sure that if a `Method` instance is being deallocate the associated `jmethodID` (if any) must not point to the deallocated instance once we are finished. Unfortunately, we can not just update the `jmethodID` values in bulk when purging an old class version - the per `InstanceKlass` jmethodID cache is present only for the main class version and contains `jmethodID` values for both the old and current method versions. 
>> 
>> ~Therefore we need to perform `jmethodID` lookup when we are about to deallocate a `Method` instance and clean up the pointer only if that `jmethodID` is pointing to the `Method` instance which is being deallocated.~
>> 
>> Therefore, we need to perform `jmethodID` lookup for each method in an old class version that is getting purged, and null out the pointer of that `jmethodID` to break the link from `jmethodID` to the method instance that is about to get deallocated.
>> 
>> _(For anyone interested, a much lengthier writeup is available in [my blog](https://jbachorik.github.io/posts/mysterious-jmethodid))_
>
> Jaroslav Bachorik has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Restrict cleanup to obsolete methods only

Just for the record Coleen's review is marked as "Review applies to [81e31dae](https://git.openjdk.org/jdk/pull/16662/files/81e31daeef4c68352368a90e3ab453ba9b33650c)" - which is not the final version.

The skara tooling does not currently support our rules but it remains as always that non-trivial Hotspot changes require two reviewers.

Thanks for the additional explanations Coleen.

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PR Comment: https://git.openjdk.org/jdk/pull/16662#issuecomment-1839721631