 static bool add_free_nid(struct f2fs_sb_info *sbi, nid_t nid, bool build)
 {
 	struct f2fs_nm_info *nm_i = NM_I(sbi);
	struct free_nid *i;
 	struct nat_entry *ne;
	int err;
 
 	/* 0 nid should not be used */
 	if (unlikely(nid == 0))
 		return false;
 
	if (build) {
		/* do not add allocated nids */
		ne = __lookup_nat_cache(nm_i, nid);
		if (ne && (!get_nat_flag(ne, IS_CHECKPOINTED) ||
				nat_get_blkaddr(ne) != NULL_ADDR))
			return false;
	}
 	i = f2fs_kmem_cache_alloc(free_nid_slab, GFP_NOFS);
 	i->nid = nid;
 	i->state = NID_NEW;
 
	if (radix_tree_preload(GFP_NOFS)) {
		kmem_cache_free(free_nid_slab, i);
		return true;
	}
 
 	spin_lock(&nm_i->nid_list_lock);
 	err = __insert_nid_to_list(sbi, i, FREE_NID_LIST, true);
 	spin_unlock(&nm_i->nid_list_lock);
 	radix_tree_preload_end();
	if (err) {
 		kmem_cache_free(free_nid_slab, i);
		return true;
	}
	return true;
 }

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.