On Thu, 23 Nov 2023 01:38:57 GMT, David Holmes <dhol...@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. > > test/hotspot/jtreg/runtime/Monitor/IterateMonitorWithDeadObjectTest.java line > 40: > >> 38: public class IterateMonitorWithDeadObjectTest { >> 39: public static native void runTestAndDetachThread(); >> 40: public static native void joinTestThread(); > > I don't think this form of the test needs to separate out the > `pthread_join()`, it can just be done in `runTestAndDetachThread` AFAICS. I > originally split it out to allow the Java code to do the GC while the native > thread was sleeping prior to detaching. All this is left-overs that I thought I had removed. I'm removing this. > test/hotspot/jtreg/runtime/Monitor/IterateMonitorWithDeadObjectTest.java line > 57: > >> 55: // - Drop the last reference to the object >> 56: // - GC to clear the weak reference to the object in the monitor >> 57: // - Detach the thread - provoke previous bug > > It also does a thread dump while the lock is held Updated ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/16783#discussion_r1403082283 PR Review Comment: https://git.openjdk.org/jdk/pull/16783#discussion_r1403082528
