task_work: remove legacy TWA_SIGNAL path

static struct callback_head work_exited; /* all we need is ->next == NULL */

/*

 * TWA_SIGNAL signaling - use TIF_NOTIFY_SIGNAL, if available, as it's faster

 * than TIF_SIGPENDING as there's no dependency on ->sighand. The latter is

 * shared for threads, and can cause contention on sighand->lock. Even for

 * the non-threaded case TIF_NOTIFY_SIGNAL is more efficient, as no locking

 * or IRQ disabling is involved for notification (or running) purposes.

 */

static void task_work_notify_signal(struct task_struct *task)

{

#if defined(TIF_NOTIFY_SIGNAL)

	set_notify_signal(task);

#else

	unsigned long flags;

	/*

	 * Only grab the sighand lock if we don't already have some

	 * task_work pending. This pairs with the smp_store_mb()

	 * in get_signal(), see comment there.

	 */

	if (!(READ_ONCE(task->jobctl) & JOBCTL_TASK_WORK) &&

	    lock_task_sighand(task, &flags)) {

		task->jobctl |= JOBCTL_TASK_WORK;

		signal_wake_up(task, 0);

		unlock_task_sighand(task, &flags);

	}

#endif

}

/**

 * task_work_add - ask the @task to execute @work->func()

 * @task: the task which should run the callback

		set_notify_resume(task);

		break;

	case TWA_SIGNAL:

		task_work_notify_signal(task);

		set_notify_signal(task);

		break;

	default:

		WARN_ON_ONCE(1);