Fix concurrent_set.c when found garbage object

Author: luke-gruber

concurrent_set.c

@@ -610,7 +610,6 @@ rb_concurrent_set_find_or_insert(VALUE *set_obj_ptr, VALUE key, void *data)
             RB_VM_LOCKING();
             goto retry;
           default:
-            // what about if hash is marked reclaimed but key is not cleared yet
             if (curr_hash != hash) {
                 goto probe_next;
             }
@@ -621,8 +620,13 @@ rb_concurrent_set_find_or_insert(VALUE *set_obj_ptr, VALUE key, void *data)
                 if (continuation) {
                     goto probe_next;
                 }
-                rbimpl_atomic_value_cas(&entry->key, raw_key, CONCURRENT_SET_EMPTY, RBIMPL_ATOMIC_RELEASE, RBIMPL_ATOMIC_RELAXED);
-                continue;
+                {
+                    VALUE prev = rbimpl_atomic_value_cas(&entry->key, raw_key, CONCURRENT_SET_EMPTY, RBIMPL_ATOMIC_RELEASE, RBIMPL_ATOMIC_RELAXED);
+                    if (prev == raw_key) {
+                        rbimpl_atomic_sub(&set->size, 1, RBIMPL_ATOMIC_RELAXED);
+                    }
+                }
+                continue; // try to reclaim same slot, because the hash is the same and it's now EMPTY
             }
 
             if (set->funcs->cmp(key, curr_key)) {