int ipc_addid(struct ipc_ids *ids, struct kern_ipc_perm *new, int size)
{
	kuid_t euid;
	kgid_t egid;
	int id;
	int next_id = ids->next_id;

	if (size > IPCMNI)
		size = IPCMNI;

	if (ids->in_use >= size)
		return -ENOSPC;

	idr_preload(GFP_KERNEL);

	spin_lock_init(&new->lock);
	new->deleted = false;
	rcu_read_lock();
	spin_lock(&new->lock);

	id = idr_alloc(&ids->ipcs_idr, new,
		       (next_id < 0) ? 0 : ipcid_to_idx(next_id), 0,
		       GFP_NOWAIT);
	idr_preload_end();
	if (id < 0) {
		spin_unlock(&new->lock);
		rcu_read_unlock();
		return id;
	}

	ids->in_use++;

	current_euid_egid(&euid, &egid);
	new->cuid = new->uid = euid;
	new->gid = new->cgid = egid;

	if (next_id < 0) {
		new->seq = ids->seq++;
		if (ids->seq > IPCID_SEQ_MAX)
			ids->seq = 0;
	} else {
		new->seq = ipcid_to_seqx(next_id);
		ids->next_id = -1;
	}

	new->id = ipc_buildid(id, new->seq);
	return id;
}
The program contains a code sequence that can run concurrently with other code, and the code sequence 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 that is operating concurrently.