From 5b87294d2fc84b7039a703cbbd02c7cf50c5b560 Mon Sep 17 00:00:00 2001
From: Peter Zhu <peter@peterzhu.ca>
Date: Sat, 3 Jan 2026 09:27:15 -0500
Subject: [PATCH 1/2] Add rb_gc_print_backtrace

---
 gc.c    | 8 ++++++++
 gc/gc.h | 1 +
 2 files changed, 9 insertions(+)

diff --git a/gc.c b/gc.c
index f77ee417c52dea..98e5712ca40214 100644
--- a/gc.c
+++ b/gc.c
@@ -129,6 +129,14 @@
 #include "builtin.h"
 #include "shape.h"
 
+// TODO: Don't export this function in modular GC, instead MMTk should figure out
+// how to combine GC thread backtrace with mutator thread backtrace.
+void
+rb_gc_print_backtrace(void)
+{
+    rb_print_backtrace(stderr);
+}
+
 unsigned int
 rb_gc_vm_lock(const char *file, int line)
 {
diff --git a/gc/gc.h b/gc/gc.h
index a5edc266e75b1a..097ddb93949a0b 100644
--- a/gc/gc.h
+++ b/gc/gc.h
@@ -108,6 +108,7 @@ MODULAR_GC_FN void rb_gc_initialize_vm_context(struct rb_gc_vm_context *context)
 MODULAR_GC_FN void rb_gc_worker_thread_set_vm_context(struct rb_gc_vm_context *context);
 MODULAR_GC_FN void rb_gc_worker_thread_unset_vm_context(struct rb_gc_vm_context *context);
 MODULAR_GC_FN void rb_gc_move_obj_during_marking(VALUE from, VALUE to);
+MODULAR_GC_FN void rb_gc_print_backtrace();
 #endif
 
 #if USE_MODULAR_GC

From a0c483fcfb9b8a2009cf21a8bce5fa2ad54d4fda Mon Sep 17 00:00:00 2001
From: Peter Zhu <peter@peterzhu.ca>
Date: Sat, 3 Jan 2026 09:51:00 -0500
Subject: [PATCH 2/2] Also output GC thread backtrace in rb_mmtk_gc_thread_bug

---
 gc/mmtk/mmtk.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/gc/mmtk/mmtk.c b/gc/mmtk/mmtk.c
index 2c447b0d389f65..e9558f906f3242 100644
--- a/gc/mmtk/mmtk.c
+++ b/gc/mmtk/mmtk.c
@@ -446,6 +446,11 @@ rb_mmtk_gc_thread_bug(const char *msg, ...)
     vsnprintf(objspace->crash_context.crash_msg, sizeof(objspace->crash_context.crash_msg), msg, args);
     va_end(args);
 
+    fprintf(stderr, "-- GC thread backtrace "
+                    "-------------------------------------------\n");
+    rb_gc_print_backtrace();
+    fprintf(stderr, "\n");
+
     rb_mmtk_resume_mutators();
 
     sleep(5);