static void io_req_task_work_add_fallback(struct io_kiocb *req,
					  task_work_func_t cb)
{
	struct io_ring_ctx *ctx = req->ctx;
	struct callback_head *head;

	init_task_work(&req->task_work, cb);
	do {
		head = READ_ONCE(ctx->exit_task_work);
		req->task_work.next = head;
	} while (cmpxchg(&ctx->exit_task_work, head, &req->task_work) != head);
}
static void io_req_task_work_add_fallback(struct io_kiocb *req,
					  task_work_func_t cb)
{
	struct io_ring_ctx *ctx = req->ctx;
	struct callback_head *head;

	init_task_work(&req->task_work, cb);
	do {
		head = READ_ONCE(ctx->exit_task_work);
		req->task_work.next = head;
	} while (cmpxchg(&ctx->exit_task_work, head, &req->task_work) != head);
}