On Thu, 23 Nov 2023 16:42:35 GMT, Jaroslav Bachorik <jbacho...@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
