Re: [PATCH 2/2] io_uring: use TWA_SIGNAL for task_work if the task isn't running

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Aug 10, 2020 at 01:21:48PM -0600, Jens Axboe wrote:

> >> Wait.. so the only change here is that you look at tsk->state, _after_
> >> doing __task_work_add(), but nothing, not the Changelog nor the comment
> >> explains this.
> >>
> >> So you're relying on __task_work_add() being an smp_mb() vs the add, and
> >> you order this against the smp_mb() in set_current_state() ?
> >>
> >> This really needs spelling out.
> > 
> > I'll update the changelog, it suffers a bit from having been reused from
> > the earlier versions. Thanks for checking!
> 
> I failed to convince myself that the existing construct was safe, so
> here's an incremental on top of that. Basically we re-check the task
> state _after_ the initial notification, to protect ourselves from the
> case where we initially find the task running, but between that check
> and when we do the notification, it's now gone to sleep. Should be
> pretty slim, but I think it's there.
> 
> Hence do a loop around it, if we're using TWA_RESUME.
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 44ac103483b6..a4ecb6c7e2b0 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -1780,12 +1780,27 @@ static int io_req_task_work_add(struct io_kiocb *req, struct callback_head *cb)
>  	 * to ensure that the issuing task processes task_work. TWA_SIGNAL
>  	 * is needed for that.
>  	 */
> -	if (ctx->flags & IORING_SETUP_SQPOLL)
> +	if (ctx->flags & IORING_SETUP_SQPOLL) {
>  		notify = 0;
> -	else if (READ_ONCE(tsk->state) != TASK_RUNNING)
> -		notify = TWA_SIGNAL;
> +	} else {
> +		bool notified = false;
>  
> -	__task_work_notify(tsk, notify);
> +		/*
> +		 * If the task is running, TWA_RESUME notify is enough. Make
> +		 * sure to re-check after we've sent the notification, as not

Could we get a clue as to why TWA_RESUME is enough when it's running? I
presume it is because we'll do task_work_run() somewhere before we
block, but having an explicit reference here might help someone new to
this make sense of it all.

> +		 * to have a race between the check and the notification. This
> +		 * only applies for TWA_RESUME, as TWA_SIGNAL is safe with a
> +		 * sleeping task
> +		 */
> +		do {
> +			if (READ_ONCE(tsk->state) != TASK_RUNNING)
> +				notify = TWA_SIGNAL;
> +			else if (notified)
> +				break;
> +			__task_work_notify(tsk, notify);
> +			notified = true;
> +		} while (notify != TWA_SIGNAL);
> +	}
>  	wake_up_process(tsk);
>  	return 0;
>  }

Would it be clearer to write it like so perhaps?

	/*
	 * Optimization; when the task is RUNNING we can do with a
	 * cheaper TWA_RESUME notification because,... <reason goes
	 * here>. Otherwise do the more expensive, but always correct
	 * TWA_SIGNAL.
	 */
	if (READ_ONCE(tsk->state) == TASK_RUNNING) {
		__task_work_notify(tsk, TWA_RESUME);
		if (READ_ONCE(tsk->state) == TASK_RUNNING)
			return;
	}
	__task_work_notify(tsk, TWA_SIGNAL);
	wake_up_process(tsk);