[pull] master from ruby:master

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)
 {

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

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);