On Wed, 1 May 2024 10:20:52 GMT, Serguei Spitsyn <sspit...@openjdk.org> wrote:
> The fix is to degrade virtual threads support in the JVM TI > `GetObjectMonitorUsage` function so that it is specified to only return an > owner when the owner is a platform thread. Also, virtual threads are not > listed in the both `waiters` and `notify_waiters` lists returned in the > `jvmtiMonitorUsage` structure. Java 19 re-specified a number of JVMTI > functions and events for virtual threads, we missed this one. > > The main motivation for degrading it now is that the object monitor > implementation is being updated to allow virtual threads unmount while owning > monitors. It would add overhead to record monitor usage when > freezing/unmount, overhead that couldn't be tied to a JVMTI capability as the > capability can be enabled at any time. > > `GetObjectMonitorUsage` was broken for 20+ years > ([8247972](https://bugs.openjdk.org/browse/JDK-8247972)) without bug reports > so it seems unlikely that the function is widely used. Degrading it to only > return an owner when the owner is a platform thread has no compatibility > impact for tooling that uses it in conjunction with `HotSpot` thread dumps or > `ThreadMXBean`. > > One other point about `GetObjectMonitorUsage` is that it pre-dates > j.u.concurrent in Java 5 so it can't be used to get a full picture of the > lock usage in a program. > > The specs of the impacted `JDWP ObjectReference.MonitorInfo` command and the > JDI `ObjectReference` `ownerThread()`, `waitingThreads()` and `entryCount()` > methods are updated to match the JVM TI spec. > > Also, please, review the related CSR and Release Note: > - CSR: [8331422](https://bugs.openjdk.org/browse/JDK-8331422): degrade > virtual thread support for GetObjectMonitorUsage > - RN: [8331465](https://bugs.openjdk.org/browse/JDK-8331465): Release Note: > degrade virtual thread support for GetObjectMonitorUsage > > Testing: > - tested impacted and updated tests locally > - tested with mach5 tiers 1-6 This pull request has now been integrated. Changeset: b890336e Author: Serguei Spitsyn <sspit...@openjdk.org> URL: https://git.openjdk.org/jdk/commit/b890336e111ea8473ae49e9992bc2fd61e716792 Stats: 188 lines in 12 files changed: 131 ins; 2 del; 55 mod 8328083: degrade virtual thread support for GetObjectMonitorUsage Reviewed-by: cjplummer, alanb ------------- PR: https://git.openjdk.org/jdk/pull/19030
