On Wed, 29 Nov 2023 11:49:31 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.~ >> >> 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 We deallocate the old Method* if nothing is referring to them (ie they're not running or being referenced for some other reason). Look at MetadataOnStackMark. The jmethodIDs to an obsolete method were a dangling pointer and we want to just null them out. The old Method* are attached to the scratch_version of the InstanceKlass so we essentially remove that and walk down to the Methods when none of them are found. ------------- PR Comment: https://git.openjdk.org/jdk/pull/16662#issuecomment-1838538568
