On Wed, 22 Nov 2023 15:00:29 GMT, Stefan Karlsson <stef...@openjdk.org> wrote:
> In the rewrites made for: > [JDK-8318757](https://bugs.openjdk.org/browse/JDK-8318757) `VM_ThreadDump > asserts in interleaved ObjectMonitor::deflate_monitor calls` > > I removed the filtering of *owned ObjectMonitors with dead objects*. The > reasoning was that you should never have an owned ObjectMonitor with a dead > object. I added an assert to check this assumption. It turns out that the > assumption was wrong *if* you use JNI to call MonitorEnter and then remove > all references to the locked object. > > The provided tests provoke this assert form: > * the JNI thread detach code > * thread dumping with locked monitors, and > * the JVMTI GetOwnedMonitorInfo API. > > While investigating this we've found that the thread detach code becomes more > correct when this filter was removed. Previously, the locked monitors never > got unlocked because the ObjectMonitor iterator never exposed these monitors > to the JNI detach code that unlocks the thread's monitors. That bug caused an > ObjectMonitor leak. So, for this case I'm leaving these ObjectMonitors > unfiltered so that we don't reintroduce the leak. > > The thread dumping case doesn't tolerate ObjectMonitor with dead objects, so > I'm filtering those in the closure that collects ObjectMonitor. Side note: We > have discussions about ways to completely rewrite this by letting each thread > have thread-local information about JNI held locks. If we have this we could > probably throw away the entire ObjectMonitorDump hashtable, and its walk of > the `_in_use_list.`. > > For GetOwnedMonitorInfo it is unclear if we should expose these weird > ObjectMonitor. If we do, then the users can detect that a thread holds a lock > with a dead object, and the code will return NULL as one of the "owned > monitors" returned. I don't think that's a good idea, so I'm filtering out > these ObjectMonitor for those calls. > > Test: the written tests with and without the fix. Tier1-Tier3, so far. This pull request has now been integrated. Changeset: 0d146361 Author: Stefan Karlsson <stef...@openjdk.org> URL: https://git.openjdk.org/jdk/commit/0d146361f27e1415fab9272de1cdde84c074c718 Stats: 326 lines in 8 files changed: 319 ins; 1 del; 6 mod 8320515: assert(monitor->object_peek() != nullptr) failed: Owned monitors should not have a dead object Reviewed-by: dholmes, ihse, sspitsyn, dcubed ------------- PR: https://git.openjdk.org/jdk/pull/16783
