On Tue, 14 Nov 2023 17:56:09 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))_ src/hotspot/share/oops/instanceKlass.cpp line 541: > 539: // The previous version will point to them so they're not totally > dangling > 540: assert (!method->on_stack(), "shouldn't be called with methods on > stack"); > 541: // Do the pointer maintenance before releasing the metadata, but > not for incomplete methods I'm confused by what you mean by method holder, which I think of as methodHandle. Or InstanceKlass is the holder of the methods. Maybe this should be more explicit that it's talking about clearing any associated jmethodIDs. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/16662#discussion_r1397970676
