linux-kernel - Re: [PATCH] task_work: cleanup notification modes

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Date:   Sat, 17 Oct 2020 01:38:58 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] task_work: cleanup notification modes

On Fri, Oct 16 2020 at 17:13, Jens Axboe wrote:
> /**
>  * task_work_add - ask the @task to execute @work->func()
>  * @task: the task which should run the callback
>  * @work: the callback to run
>  * @notify: how to notify the targeted task
>  *
>  * Queue @work for task_work_run() below and notify the @task if @notify
>  * is @TWA_RESUME or @TWA_SIGNAL. @TWA_SIGNAL work like signals, in that the

s/the//

>  * it will interrupt the targeted task and run the task_work. @TWA_RESUME
>  * work is run only when the task exits the kernel and returns to user mode.
>  * Fails if the @task is exiting/exited and thus it can't process this @work.
>  * Otherwise @work->func() will be called when the @task returns from kernel
>  * mode or exits.

Yes, that makes a lot more sense.

What's still lacking is a description of the return value and how to act
upon it.

Most of the call sites ignore it, some are acting upon it but I can't
make any sense of these actions:

fs/io_uring.c-	notify = 0;
fs/io_uring.c-	if (!(ctx->flags & IORING_SETUP_SQPOLL) && twa_signal_ok)
fs/io_uring.c-		notify = TWA_SIGNAL;
fs/io_uring.c-
fs/io_uring.c:	ret = task_work_add(tsk, &req->task_work, notify);
fs/io_uring.c-	if (!ret)
fs/io_uring.c-		wake_up_process(tsk);

???

fs/io_uring.c-	if (unlikely(ret)) {
fs/io_uring.c-		struct task_struct *tsk;
fs/io_uring.c-
fs/io_uring.c-		init_task_work(&req->task_work, io_req_task_cancel);
fs/io_uring.c-		tsk = io_wq_get_task(req->ctx->io_wq);
fs/io_uring.c:		task_work_add(tsk, &req->task_work, 0);
fs/io_uring.c-		wake_up_process(tsk);

yet more magic wakeup.

fs/io_uring.c-
fs/io_uring.c-	init_task_work(&req->task_work, io_req_task_submit);
fs/io_uring.c-	percpu_ref_get(&req->ctx->refs);
fs/io_uring.c-
fs/io_uring.c-	/* submit ref gets dropped, acquire a new one */
fs/io_uring.c-	refcount_inc(&req->refs);
fs/io_uring.c:	ret = io_req_task_work_add(req, true);
fs/io_uring.c-	if (unlikely(ret)) {
fs/io_uring.c-		struct task_struct *tsk;
fs/io_uring.c-
fs/io_uring.c-		/* queue just for cancelation */
fs/io_uring.c-		init_task_work(&req->task_work, io_req_task_cancel);
fs/io_uring.c-		tsk = io_wq_get_task(req->ctx->io_wq);
fs/io_uring.c:		task_work_add(tsk, &req->task_work, 0);
fs/io_uring.c-		wake_up_process(tsk);

Ditto. Why the heck is this wakeup making any sense? The initial
task_work_add() within io_req_task_work_add() failed already ...

fs/io_uring.c:	ret = io_req_task_work_add(req, twa_signal_ok);
fs/io_uring.c-	if (unlikely(ret)) {
fs/io_uring.c-		struct task_struct *tsk;
fs/io_uring.c-
fs/io_uring.c-		WRITE_ONCE(poll->canceled, true);
fs/io_uring.c-		tsk = io_wq_get_task(req->ctx->io_wq);
fs/io_uring.c:		task_work_add(tsk, &req->task_work, 0);
fs/io_uring.c-		wake_up_process(tsk);

...

Thanks,

        tglx