Re: [PATCH v4 02/39] task_work: Fix TWA_NMI_CURRENT race with __schedule()

On Wed, Jan 22, 2025 at 01:42:28PM +0100, Peter Zijlstra wrote:
> So I'm a little confused, isn't something like this sufficient?
> 
> If we hit before schedule(), all just works as expected, if we hit after
> schedule(), the task will already have the TIF flag set, and we'll hit
> the return to user path once it gets scheduled again.
> 
> ---
> diff --git a/kernel/task_work.c b/kernel/task_work.c
> index c969f1f26be5..155549c017b2 100644
> --- a/kernel/task_work.c
> +++ b/kernel/task_work.c
> @@ -9,7 +9,12 @@ 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);
> +	/*
> +	 * no-op IPI
> +	 *
> +	 * TWA_NMI_CURRENT will already have set the TIF flag, all
> +	 * this interrupt does it tickle the return-to-user path.
> +	 */
>  }
>  static DEFINE_PER_CPU(struct irq_work, irq_work_NMI_resume) =
>  	IRQ_WORK_INIT_HARD(task_work_set_notify_irq);
> @@ -98,6 +103,7 @@ int task_work_add(struct task_struct *task, struct callback_head *work,
>  		break;
>  #ifdef CONFIG_IRQ_WORK
>  	case TWA_NMI_CURRENT:
> +		set_tsk_thread_flag(current, TIF_NOTIFY_RESUME);
>  		irq_work_queue(this_cpu_ptr(&irq_work_NMI_resume));
>  		break;
>  #endif

Yeah, that looks so much better...

The self-IPI is only needed when the NMI happened in user space, right?
Would it make sense to have an optimized version of that?

-- 
Josh