## Summary After having switched to the new nonblocking GC in production we've been experiencing frequent segfaults. I've been able to extract the following stack trace from a core dump[^1]: [^1]: I'm sadly unable to share the full core dump since it likely contains personal information of third parties I'm legally unable to disclose ``` #0 isAlive (p=0x4214d682b0) at rts/sm/GCAux.c:96 #1 0x000000000e45de5a in tidyWeakList (gen=<optimized out>) at rts/sm/MarkWeak.c:266 #2 0x000000000e45e119 in traverseWeakPtrList (dead_weak_ptr_list=dead_weak_ptr_list@entry=0x7f156d7f9760, resurrected_threads=resurrected_threads@entry=0x7f156d7f9768) at rts/sm/MarkWeak.c:124 #3 0x000000000e43f47d in GarbageCollect (collect_gen=<optimized out>, collect_gen@entry=1, do_heap_census=do_heap_census@entry=false, deadlock_detect=deadlock_detect@entry=false, gc_type=gc_type@entry=2, cap=0x12194720, cap@entry=0x121dc180, idle_cap=idle_cap@entry=0x7f154801c220) at rts/sm/GC.c:456 #4 0x000000000e431f7e in scheduleDoGC (pcap=pcap@entry=0x7f156d7f99f0, task=task@entry=0x7f1578000b70, force_major=force_major@entry=false, deadlock_detect=deadlock_detect@entry=false) at rts/Schedule.c:1853 #5 0x000000000e432db8 in schedule (initialCapability=initialCapability@entry=0x12194720, task=task@entry=0x7f1578000b70) at rts/Schedule.c:565 #6 0x000000000e433e1c in scheduleWorker (cap=cap@entry=0x12194720, task=task@entry=0x7f1578000b70) at rts/Schedule.c:2617 #7 0x000000000e4300d3 in workerStart (task=0x7f1578000b70) at rts/Task.c:445 #8 0x00007f1595ff06db in start_thread (arg=0x7f156d7fa700) at pthread_create.c:463 #9 0x00007f1595489a3f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95 ``` I'm sure I can spare some time to further dissect the core dump (with some guidance) if there is interest to do so. ## Steps to reproduce I've been unable to reproduce the issue during testing or running in a local development environment; I assume it's triggered by the higher load of the production environment. The project is a ~60k LOC Yesod-based and database-heavy webapplication. ## Expected behavior An absence of segfaults ;) ## Environment * GHC version used: 8.10.2 * Operating System: Ubuntu 18.04.5 LTS (Bionic Beaver) `Linux 4.15.0-112-generic #113-Ubuntu SMP Thu Jul 9 23:41:39 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux`