RFR: 8313816: Accessing jmethodID might lead to spurious crashes

[David Holmes]

On Tue, 14 Nov 2023 17:56:09 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))_

src/hotspot/share/oops/instanceKlass.cpp line 531:

> 529: 
> 530: void InstanceKlass::deallocate_methods(ClassLoaderData* loader_data,
> 531:                                        Array<Method*>* methods, InstanceKlass* klass) {

An explicit boolean parameter would be cleaner/clearer.

src/hotspot/share/oops/instanceKlass.cpp line 542:

> 540:       if (klass) {
> 541:         jmethodID jmid = method->find_jmethod_id_or_null();
> 542:         // Do the pointer maintenance before releasing the metadata, just in case

I assume there should be a period after 'case`. But just in case of what?

src/hotspot/share/oops/instanceKlass.cpp line 549:

> 547:         if (jmid != nullptr && *((Method**)jmid) == method) {
> 548:           *((Method**)jmid) = nullptr;
> 549:         }

This should be abstracted behind a utility function e.g. `method->clear_jmethod_id()`.

src/hotspot/share/oops/method.cpp line 2277:

> 2275:   }
> 2276: }
> 2277: 

Can this race with redefinition?

src/hotspot/share/oops/method.hpp line 730:

> 728:   // so handles are not used to avoid deadlock.
> 729:   jmethodID find_jmethod_id_or_null()               {
> 730:     return method_holder() != nullptr ? method_holder()->jmethod_id_or_null(this) : nullptr;

If `method_holder()` is null at this point what does that mean for the lifecycle of the Method?

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PR Review Comment: https://git.openjdk.org/jdk/pull/16662#discussion_r1393663791
PR Review Comment: https://git.openjdk.org/jdk/pull/16662#discussion_r1393664277
PR Review Comment: https://git.openjdk.org/jdk/pull/16662#discussion_r1393672300
PR Review Comment: https://git.openjdk.org/jdk/pull/16662#discussion_r1395072721
PR Review Comment: https://git.openjdk.org/jdk/pull/16662#discussion_r1395071297