ns_subclasses refcount accesses need to be atomic (ruby#15083)

We were seeing errors like:

```
* thread #8, stop reason = EXC_BAD_ACCESS (code=1, address=0x803)
  * frame #0: 0x00000001001fe944 ruby`rb_st_lookup(tab=0x00000000000007fb, key=1, value=0x00000001305b7490) at st.c:1066:22
    frame #1: 0x000000010002d658 ruby`remove_class_from_subclasses [inlined] class_get_subclasses_for_ns(tbl=0x00000000000007fb, ns_id=1) at class.c:604:9
    frame #2: 0x000000010002d650 ruby`remove_class_from_subclasses(tbl=0x00000000000007fb, ns_id=1, klass=4754039232) at class.c:620:34
    frame #3: 0x000000010002c8a8 ruby`rb_class_classext_free_subclasses(ext=0x000000011b5ce1d8, klass=4754039232, replacing=<unavailable>) at class.c:700:9
    frame #4: 0x000000010002c760 ruby`rb_class_classext_free(klass=4754039232, ext=0x000000011b5ce1d8, is_prime=true) at class.c:105:5
    frame #5: 0x00000001000e770c ruby`classext_free(ext=<unavailable>, is_prime=<unavailable>, namespace=<unavailable>, arg=<unavailable>) at gc.c:1231:5 [artificial]
    frame #6: 0x000000010002d178 ruby`rb_class_classext_foreach(klass=<unavailable>, func=(ruby`classext_free at gc.c:1228), arg=0x00000001305b75c0) at class.c:518:5
    frame #7: 0x00000001000e745c ruby`rb_gc_obj_free(objspace=0x000000012500c400, obj=4754039232) at gc.c:1282:9
    frame #8: 0x00000001000e70d4 ruby`gc_sweep_plane(objspace=0x000000012500c400, heap=<unavailable>, p=4754039232, bitset=4095, ctx=0x00000001305b76e8) at default.c:3482:21
    frame #9: 0x00000001000e6e9c ruby`gc_sweep_page(objspace=0x000000012500c400, heap=0x000000012500c540, ctx=0x00000001305b76e8) at default.c:3567:13
    frame #10: 0x00000001000e51d0 ruby`gc_sweep_step(objspace=0x000000012500c400, heap=0x000000012500c540) at default.c:3848:9
    frame #11: 0x00000001000e1880 ruby`gc_continue [inlined] gc_sweep_continue(objspace=0x000000012500c400, sweep_heap=0x000000012500c540) at default.c:3931:13
    frame #12: 0x00000001000e1754 ruby`gc_continue(objspace=0x000000012500c400, heap=0x000000012500c540) at default.c:2037:9
    frame #13: 0x00000001000e10bc ruby`newobj_cache_miss [inlined] heap_prepare(objspace=0x000000012500c400, heap=0x000000012500c540) at default.c:2056:5
    frame #14: 0x00000001000e1074 ruby`newobj_cache_miss [inlined] heap_next_free_page(objspace=0x000000012500c400, heap=0x000000012500c540) at default.c:2280:9
    frame #15: 0x00000001000e106c ruby`newobj_cache_miss(objspace=0x000000012500c400, cache=0x0000600001b00300, heap_idx=2, vm_locked=false) at default.c:2387:38
    frame #16: 0x00000001000e0d28 ruby`newobj_alloc(objspace=<unavailable>, cache=<unavailable>, heap_idx=<unavailable>, vm_locked=<unavailable>) at default.c:2411:15 [artificial]
    frame #17: 0x00000001000d7214 ruby`newobj_of [inlined] rb_gc_impl_new_obj(objspace_ptr=<unavailable>, cache_ptr=<unavailable>, klass=<unavailable>, flags=<unavailable>, wb_protected=<unavailable>, alloc_size=<unavailable>) at default.c:2490:15
    frame #18: 0x00000001000d719c ruby`newobj_of(cr=<unavailable>, klass=4313971728, flags=258, wb_protected=<unavailable>, size=<unavailable>) at gc.c:995:17
    frame #19: 0x00000001000d73ec ruby`rb_wb_protected_newobj_of(ec=<unavailable>, klass=<unavailable>, flags=<unavailable>, size=<unavailable>) at gc.c:1044:12 [artificial]
    frame #20: 0x0000000100032d34 ruby`class_alloc0(type=<unavailable>, klass=4313971728, namespaceable=<unavailable>) at class.c:803:5
```

Author: luke-gruber

class.c

@@ -690,7 +690,7 @@ rb_class_classext_free_subclasses(rb_classext_t *ext, VALUE klass, bool replacin
     }
     VM_ASSERT(
         rb_ns_subclasses_ref_count(anchor->ns_subclasses) > 0,
-        "ns_subclasses refcount (%p) %ld", anchor->ns_subclasses, rb_ns_subclasses_ref_count(anchor->ns_subclasses));
+        "ns_subclasses refcount (%p) %d", anchor->ns_subclasses, rb_ns_subclasses_ref_count(anchor->ns_subclasses));
     st_delete(tbl, &ns_id, NULL);
     rb_ns_subclasses_ref_dec(anchor->ns_subclasses);
     xfree(anchor);

internal/class.h

@@ -28,29 +28,29 @@
 #endif
 
 struct rb_ns_subclasses {
-    long refcount;
+    rb_atomic_t refcount;
     struct st_table *tbl;
 };
 typedef struct rb_ns_subclasses rb_ns_subclasses_t;
 
-static inline long
+static inline rb_atomic_t
 rb_ns_subclasses_ref_count(rb_ns_subclasses_t *ns_sub)
 {
-    return ns_sub->refcount;
+    return ATOMIC_LOAD_RELAXED(ns_sub->refcount);
 }
 
 static inline rb_ns_subclasses_t *
 rb_ns_subclasses_ref_inc(rb_ns_subclasses_t *ns_sub)
 {
-    ns_sub->refcount++;
+    RUBY_ATOMIC_FETCH_ADD(ns_sub->refcount, 1);
     return ns_sub;
 }
 
 static inline void
 rb_ns_subclasses_ref_dec(rb_ns_subclasses_t *ns_sub)
 {
-    ns_sub->refcount--;
-    if (ns_sub->refcount == 0) {
+    rb_atomic_t was = RUBY_ATOMIC_FETCH_SUB(ns_sub->refcount, 1);
+    if (was == 1) {
         st_free_table(ns_sub->tbl);
         xfree(ns_sub);
     }

test/ruby/test_class.rb

@@ -887,4 +887,19 @@ def test_method_table_assignment_just_after_class_init
       class C; end
     end;
   end
+
+  def test_subclasses_refcount_in_ractors
+    assert_ractor "#{<<~"begin;"}\n#{<<~'end;'}"
+    begin;
+      rs = []
+      8.times do
+        rs << Ractor.new do
+          5_000.times do
+            Class.new
+          end
+        end
+      end
+      rs.each(&:join)
+    end;
+  end
 end