task_work: make TWA_NMI_CURRENT handling conditional on IRQ_WORK

The TWA_NMI_CURRENT handling very much depends on IRQ_WORK, but that
isn't universally enabled everywhere.

Maybe the IRQ_WORK infrastructure should just be unconditional - x86
ends up indirectly enabling it through unconditionally enabling
PERF_EVENTS, for example.  But it also gets enabled by having SMP
support, or even if you just have PRINTK enabled.

But in the meantime TWA_NMI_CURRENT causes tons of build failures on
various odd minimal configs.  Which did show up in linux-next, but
despite that nobody bothered to fix it or even inform me until -rc1 was
out.

Fixes: 466e4d80 ("task_work: Add TWA_NMI_CURRENT as an additional notify mode")
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Reported-by: kernelci.org bot <bot@kernelci.org>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/task_work.c

@@ -6,12 +6,14 @@
 
 static struct callback_head work_exited; /* all we need is ->next == NULL */
 
+#ifdef CONFIG_IRQ_WORK
 static void task_work_set_notify_irq(struct irq_work *entry)
 {
 	test_and_set_tsk_thread_flag(current, TIF_NOTIFY_RESUME);
 }
 static DEFINE_PER_CPU(struct irq_work, irq_work_NMI_resume) =
 	IRQ_WORK_INIT_HARD(task_work_set_notify_irq);
+#endif
 
 /**
  * task_work_add - ask the @task to execute @work->func()
@@ -57,6 +59,8 @@ int task_work_add(struct task_struct *task, struct callback_head *work,
 	if (notify == TWA_NMI_CURRENT) {
 		if (WARN_ON_ONCE(task != current))
 			return -EINVAL;
+		if (!IS_ENABLED(CONFIG_IRQ_WORK))
+			return -EINVAL;
 	} else {
 		/* record the work call stack in order to print it in KASAN reports */
 		kasan_record_aux_stack(work);
@@ -81,9 +85,11 @@ int task_work_add(struct task_struct *task, struct callback_head *work,
 	case TWA_SIGNAL_NO_IPI:
 		__set_notify_signal(task);
 		break;
+#ifdef CONFIG_IRQ_WORK
 	case TWA_NMI_CURRENT:
 		irq_work_queue(this_cpu_ptr(&irq_work_NMI_resume));
 		break;
+#endif
 	default:
 		WARN_ON_ONCE(1);
 		break;