Stuck in lusearch

[qinsoon]

@wks reported this issue in mmtk/mmtk-core#1367 (comment). This is also observed in #314 to test mmtk/mmtk-core#1364.

What appeared to happen is that we triggered GC very frequently for lusearch, and failed to bring all the mutator threads to stop after triggering a GC. MMTk core was waiting for mutators to stop.

The chance to trigger this issue is not rare, as we may see it with any of the dacapo 2006 test scripts.

/home/runner/work/mmtk-core/mmtk-core/mmtk-openjdk/.github/scripts/../../repos/openjdk/build/linux-x86_64-normal-server-fastdebug/jdk/bin/java -XX:+UseThirdPartyHeap -server -XX:MetaspaceSize=100M -Xms32M -Xmx32M -jar /home/runner/work/mmtk-core/mmtk-core/mmtk-openjdk/.github/scripts/../../dacapo/dacapo-2006-10-MR2.jar lusearch
[2025-08-12T06:43:46Z INFO  mmtk::memory_manager] Initialized MMTk with Immix (FixedHeapSize(33554432))
===== DaCapo lusearch starting =====
[2025-08-12T06:43:47Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8205/8192 pages)
[2025-08-12T06:43:47Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8213/8192 pages)
[2025-08-12T06:43:47Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8221/8192 pages)
[2025-08-12T06:43:47Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8256/8192 pages)
[2025-08-12T06:43:47Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8256/8192 pages)
[2025-08-12T06:43:47Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8264/8192 pages)
[2025-08-12T06:43:47Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8229/8192 pages)
[2025-08-12T06:43:47Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8240/8192 pages)
[2025-08-12T06:43:47Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8248/8192 pages)
[2025-08-12T06:43:47Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8256/8192 pages)
[2025-08-12T06:43:47Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8264/8192 pages)
[2025-08-12T06:43:47Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8273/8192 pages)
[2025-08-12T06:43:47Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8205/8192 pages)
[2025-08-12T06:43:47Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8248/8192 pages)
[2025-08-12T06:43:47Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8248/8192 pages)
[2025-08-12T06:43:47Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8213/8192 pages)
[2025-08-12T06:43:47Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8229/8192 pages)
[2025-08-12T06:43:47Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8221/8192 pages)
[2025-08-12T06:43:47Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8205/8192 pages)
[2025-08-12T06:43:47Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8240/8192 pages)
[2025-08-12T06:43:47Z INFO  mmtk::policy::immix::defrag] Defrag: false
[2025-08-12T06:43:47Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8221/8192 pages)
[2025-08-12T06:43:47Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8240/8192 pages)
[2025-08-12T06:43:47Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8213/8192 pages)
[2025-08-12T06:43:47Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8213/8192 pages)
[2025-08-12T06:43:47Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8221/8192 pages)
[2025-08-12T06:43:47Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8213/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8217/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8217/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8209/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8209/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8217/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8217/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8225/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8217/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::scheduler::scheduler] End of GC (4298/8192 pages, took 49 ms)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8196/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8375/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8204/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8212/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8220/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8229/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8238/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8350/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8341/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8341/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8254/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8265/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8273/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8281/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8290/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8299/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8307/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8315/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8323/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8333/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8265/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8254/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8254/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8254/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8341/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8350/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8358/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8367/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8375/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8350/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::policy::immix::defrag] Defrag: false
[2025-08-12T06:43:58Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8246/8192 pages)
[2025-08-12T06:43:58Z INFO  mmtk::scheduler::scheduler] End of GC (4271/8192 pages, took 48 ms)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8196/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8204/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8196/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8196/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8196/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8196/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::policy::immix::defrag] Defrag: false
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8204/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8212/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8204/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8229/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8229/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8246/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8246/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8220/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8220/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8238/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8212/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8212/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8212/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8212/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8212/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8212/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8212/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8220/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8220/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8238/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8220/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8220/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8196/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::scheduler::scheduler] End of GC (4178/8192 pages, took 48 ms)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8196/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8204/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8212/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8367/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8367/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8229/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8238/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8246/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8358/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8265/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8273/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8281/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8290/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8333/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8323/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8307/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8315/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8323/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8333/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8341/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8341/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8350/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8358/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8367/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8375/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8375/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::policy::immix::defrag] Defrag: false
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8220/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8254/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8299/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8333/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::scheduler::scheduler] End of GC (4151/8192 pages, took 48 ms)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8194/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8194/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8202/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8202/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::policy::immix::defrag] Defrag: false
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8228/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8210/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8202/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8228/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8236/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8219/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8219/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8228/8192 pages)
[2025-08-12T06:43:59Z INFO  mmtk::util::heap::gc_trigger] [POLL] immix: Triggering collection (8236/8192 pages)

[qinsoon]

This issue can be reproduced on CI runners at a chance of 1/5 or 1/10, but I can't reproduce this locally on moma machines.

EDIT: The following is not right. The reference handler called wait which should release the lock.
It is caused by deadlock on Heap_lock.

The deadlock happens in a GC when an MMTk GC worker is calling mmtk_enqueue_references, and tries to acquire the Heap_lock, with the following stack trace.

#0  __futex_abstimed_wait_common64 (private=0, cancel=true, abstime=0x0, op=393, expected=0, futex_word=0x7f0b641dc380) at ./nptl/futex-internal.c:57
#1  __futex_abstimed_wait_common (cancel=true, private=0, abstime=0x0, clockid=0, expected=0, futex_word=0x7f0b641dc380) at ./nptl/futex-internal.c:87
#2  __GI___futex_abstimed_wait_cancelable64 (futex_word=futex_word@entry=0x7f0b641dc380, expected=expected@entry=0, clockid=clockid@entry=0, abstime=abstime@entry=0x0, private=private@entry=0) at ./nptl/futex-internal.c:139
#3  0x00007f0b6ea93a41 in __pthread_cond_wait_common (abstime=0x0, clockid=0, mutex=0x7f0b641dc330, cond=0x7f0b641dc358) at ./nptl/pthread_cond_wait.c:503
#4  ___pthread_cond_wait (cond=0x7f0b641dc358, mutex=0x7f0b641dc330) at ./nptl/pthread_cond_wait.c:627
#5  0x00007f0b6de399cb in os::PlatformEvent::park() () from /home/runner/work/mmtk-openjdk/mmtk-openjdk/bundles/jdk/lib/server/libjvm.so
#6  0x00007f0b6dd80570 in Monitor::ILock(Thread*) () from /home/runner/work/mmtk-openjdk/mmtk-openjdk/bundles/jdk/lib/server/libjvm.so
#7  0x00007f0b6dd83f0b in Monitor::lock(Thread*) () from /home/runner/work/mmtk-openjdk/mmtk-openjdk/bundles/jdk/lib/server/libjvm.so
#8  0x00007f0b6dd46dc4 in mmtk_enqueue_references(void**, unsigned long) () from /home/runner/work/mmtk-openjdk/mmtk-openjdk/bundles/jdk/lib/server/libjvm.so
#9  0x00007f0b6bfe1ed5 in mmtk_openjdk::reference_glue::{impl#0}::enqueue_references<false> (references=..., _tls=...) at src/reference_glue.rs:24
#10 0x00007f0b6bd0007b in mmtk::util::reference_processor::ReferenceProcessor::enqueue<mmtk_openjdk::OpenJDK<false>> (self=0x7f0b6c6b8638 <<mmtk_openjdk::SINGLETON_UNCOMPRESSED as core::ops::deref::Deref>::deref::__stability::LAZY+224>, tls=...) at /home/runner/.cargo/git/checkouts/mmtk-core-89cdc7bf360cce7f/cabdd66/src/util/reference_processor.rs:282
#11 0x00007f0b6bd1adb0 in mmtk::util::reference_processor::ReferenceProcessors::enqueue_refs<mmtk_openjdk::OpenJDK<false>> (self=0x7f0b6c6b85d8 <<mmtk_openjdk::SINGLETON_UNCOMPRESSED as core::ops::deref::Deref>::deref::__stability::LAZY+128>, tls=...) at /home/runner/.cargo/git/checkouts/mmtk-core-89cdc7bf360cce7f/cabdd66/src/util/reference_processor.rs:61
#12 0x00007f0b6bcdf843 in mmtk::util::reference_processor::{impl#12}::do_work<mmtk_openjdk::OpenJDK<false>> (self=0x1, worker=0x7f0b641d5b90, mmtk=0x7f0b6c6b8558 <<mmtk_openjdk::SINGLETON_UNCOMPRESSED as core::ops::deref::Deref>::deref::__stability::LAZY>) at /home/runner/.cargo/git/checkouts/mmtk-core-89cdc7bf360cce7f/cabdd66/src/util/reference_processor.rs:589
#13 0x00007f0b6bd2104d in mmtk::scheduler::work::GCWork::do_work_with_stat<mmtk::util::reference_processor::RefEnqueue<mmtk_openjdk::OpenJDK<false>>, mmtk_openjdk::OpenJDK<false>> (self=0x1, worker=0x7f0b641d5b90, mmtk=0x7f0b6c6b8558 <<mmtk_openjdk::SINGLETON_UNCOMPRESSED as core::ops::deref::Deref>::deref::__stability::LAZY>)
    at /home/runner/.cargo/git/checkouts/mmtk-core-89cdc7bf360cce7f/cabdd66/src/scheduler/work.rs:45
#14 0x00007f0b6bd4dc2f in mmtk::scheduler::worker::GCWorker<mmtk_openjdk::OpenJDK<false>>::run<mmtk_openjdk::OpenJDK<false>> (self=0x7f0b641d5b90, tls=..., mmtk=0x7f0b6c6b8558 <<mmtk_openjdk::SINGLETON_UNCOMPRESSED as core::ops::deref::Deref>::deref::__stability::LAZY>) at /home/runner/.cargo/git/checkouts/mmtk-core-89cdc7bf360cce7f/cabdd66/src/scheduler/worker.rs:257
#15 0x00007f0b6c018e65 in mmtk::memory_manager::start_worker<mmtk_openjdk::OpenJDK<false>> (mmtk=0x7f0b6c6b8558 <<mmtk_openjdk::SINGLETON_UNCOMPRESSED as core::ops::deref::Deref>::deref::__stability::LAZY>, tls=..., worker=0x7f0b641d5b90) at /home/runner/.cargo/git/checkouts/mmtk-core-89cdc7bf360cce7f/cabdd66/src/memory_manager.rs:559
#16 0x00007f0b6beae7c2 in mmtk_openjdk::api::start_worker (tls=..., worker=0x7f0b641d5b90) at src/api.rs:214
#17 0x00007f0b6e128fcb in Thread::call_run() () from /home/runner/work/mmtk-openjdk/mmtk-openjdk/bundles/jdk/lib/server/libjvm.so
#18 0x00007f0b6de29c06 in thread_native_entry(Thread*) () from /home/runner/work/mmtk-openjdk/mmtk-openjdk/bundles/jdk/lib/server/libjvm.so
#19 0x00007f0b6ea94ac3 in start_thread (arg=<optimized out>) at ./nptl/pthread_create.c:442
#20 0x00007f0b6eb26850 in clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:81

There is a Java ReferenceHandler thread that gets blocked for GC while holding Heap_lock in JVM_WaitForReferencePendingList in:

#0  __futex_abstimed_wait_common64 (private=0, cancel=true, abstime=0x0, op=393, expected=0, futex_word=0x7f0b641aa384) at ./nptl/futex-internal.c:57
#1  __futex_abstimed_wait_common (cancel=true, private=0, abstime=0x0, clockid=0, expected=0, futex_word=0x7f0b641aa384) at ./nptl/futex-internal.c:87
#2  __GI___futex_abstimed_wait_cancelable64 (futex_word=futex_word@entry=0x7f0b641aa384, expected=expected@entry=0, clockid=clockid@entry=0, abstime=abstime@entry=0x0, private=private@entry=0) at ./nptl/futex-internal.c:139
#3  0x00007f0b6ea93a41 in __pthread_cond_wait_common (abstime=0x0, clockid=0, mutex=0x7f0b641aa330, cond=0x7f0b641aa358) at ./nptl/pthread_cond_wait.c:503
#4  ___pthread_cond_wait (cond=0x7f0b641aa358, mutex=0x7f0b641aa330) at ./nptl/pthread_cond_wait.c:627
#5  0x00007f0b6de399cb in os::PlatformEvent::park() () from /home/runner/work/mmtk-openjdk/mmtk-openjdk/bundles/jdk/lib/server/libjvm.so
#6  0x00007f0b6dd8062c in Monitor::ILock(Thread*) () from /home/runner/work/mmtk-openjdk/mmtk-openjdk/bundles/jdk/lib/server/libjvm.so
#7  0x00007f0b6dd83179 in Monitor::lock_without_safepoint_check() () from /home/runner/work/mmtk-openjdk/mmtk-openjdk/bundles/jdk/lib/server/libjvm.so
#8  0x00007f0b6dfa32fe in SafepointSynchronize::block(JavaThread*) () from /home/runner/work/mmtk-openjdk/mmtk-openjdk/bundles/jdk/lib/server/libjvm.so
#9  0x00007f0b6dfaaa9d in SafepointMechanism::block_if_requested_slow(JavaThread*) () from /home/runner/work/mmtk-openjdk/mmtk-openjdk/bundles/jdk/lib/server/libjvm.so
#10 0x00007f0b6dd84971 in Monitor::wait(bool, long, bool) () from /home/runner/work/mmtk-openjdk/mmtk-openjdk/bundles/jdk/lib/server/libjvm.so
#11 0x00007f0b6d92002e in JVM_WaitForReferencePendingList () from /home/runner/work/mmtk-openjdk/mmtk-openjdk/bundles/jdk/lib/server/libjvm.so
#12 0x00007f0b4c61254f in ?? ()
#13 0x00007f0b6b020858 in ?? ()
#14 0x00007f0b6b020808 in ?? ()
#15 0x0000000000000000 in ?? ()