RFR: 8313816: Accessing jmethodID might lead to spurious crashes [v8]

[David Holmes]

On Thu, 23 Nov 2023 16:42:35 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.
>> 
>> _(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:
> 
>   Reinstate mistakenly deleted comment

This has gotten a lot more complicated. All I was suggesting was if this:

if (with_method_holders) {
  method->clear_jmethod_id();
}

could be changed to 

if (method->method_holder() == nullptr) {
  method->clear_jmethod_id();
}

Now I'm not at all sure what you are doing.

src/hotspot/share/oops/instanceKlass.hpp line 1084:

> 1082:   inline void release_set_methods_jmethod_ids(jmethodID* jmeths);
> 1083:   // Used to explicitly clear jmethodIDs for the contained methods
> 1084:   // This is required for JDK-8313816 but should not be used otherwise!

This comment is not helpful - what does it actually mean? Referring to a bugid doesn't help.

-------------

PR Review: https://git.openjdk.org/jdk/pull/16662#pullrequestreview-1747340307
PR Review Comment: https://git.openjdk.org/jdk/pull/16662#discussion_r1403962272