2 files changed, 32 insertions, 13 deletions
diff --git a/bootstraptest/test_ractor.rb b/bootstraptest/test_ractor.rb
index 19a6e16302..5bf697a5d0 100644
--- a/bootstraptest/test_ractor.rb
+++ b/bootstraptest/test_ractor.rb
@@ -2259,3 +2259,20 @@ rescue NotImplementedError
   :ok
 end
 }
+
+# Creating classes inside of Ractors
+# [Bug #18119]
+assert_equal 'ok', %q{
+  workers = (0...8).map do
+    Ractor.new do
+      loop do
+        100.times.map { Class.new }
+        Ractor.yield nil
+      end
+    end
+  end
+
+  100.times { Ractor.select(*workers) }
+
+  'ok'
+}
diff --git a/class.c b/class.c
index dca3bacf7e..7d7304faf6 100644
--- a/class.c
+++ b/class.c
@@ -83,23 +83,25 @@ RUBY_EXTERN rb_serial_t ruby_vm_global_cvar_state;
 static rb_subclass_entry_t *
 push_subclass_entry_to_list(VALUE super, VALUE klass)
 {
-    rb_subclass_entry_t *entry, *head;
-
-    entry = ZALLOC(rb_subclass_entry_t);
+    rb_subclass_entry_t *entry = ZALLOC(rb_subclass_entry_t);
     entry->klass = klass;
 
-    head = RCLASS_SUBCLASSES(super);
-    if (!head) {
-        head = ZALLOC(rb_subclass_entry_t);
-        RCLASS_SUBCLASSES(super) = head;
-    }
-    entry->next = head->next;
-    entry->prev = head;
+    RB_VM_LOCK_ENTER();
+    {
+        rb_subclass_entry_t *head = RCLASS_SUBCLASSES(super);
+        if (!head) {
+            head = ZALLOC(rb_subclass_entry_t);
+            RCLASS_SUBCLASSES(super) = head;
+        }
+        entry->next = head->next;
+        entry->prev = head;
 
-    if (head->next) {
-        head->next->prev = entry;
+        if (head->next) {
+            head->next->prev = entry;
+        }
+        head->next = entry;
     }
-    head->next = entry;
+    RB_VM_LOCK_LEAVE();
 
     return entry;
 }