linux-kernel - Re: [PATCH 3/3] task_work: use TIF

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Message-ID: <871rigejb8.fsf@nanos.tec.linutronix.de>
Date:   Fri, 02 Oct 2020 17:31:07 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Oleg Nesterov <oleg@...hat.com>, Jens Axboe <axboe@...nel.dk>
Cc:     linux-kernel@...r.kernel.org, io-uring@...r.kernel.org,
        peterz@...radead.org
Subject: Re: [PATCH 3/3] task_work: use TIF_TASKWORK if available

On Fri, Oct 02 2020 at 17:14, Oleg Nesterov wrote:
> Heh. To be honest I don't really like 1-2 ;)

I do not like any of this :)

> So I think that if we are going to add TIF_TASKWORK we should generalize
> this logic and turn it into TIF_NOTIFY_SIGNAL. Similar to TIF_NOTIFY_RESUME
> but implies signal_pending().
>
> IOW, something like
>
> 	void set_notify_signal(task)
> 	{
> 		if (!test_and_set_tsk_thread_flag(task, TIF_NOTIFY_SIGNAL)) {
> 			if (!wake_up_state(task, TASK_INTERRUPTIBLE))
> 				kick_process(t);
> 		}
> 	}
>
> 	// called by exit_to_user_mode_loop() if ti_work & _TIF_NOTIFY_SIGNAL
> 	void tracehook_notify_signal(regs)
> 	{
> 		clear_thread_flag(TIF_NOTIFY_SIGNAL);
> 		smp_mb__after_atomic();
> 		if (unlikely(current->task_works))
> 			task_work_run();
> 	}
>
> This way task_work_run() doesn't need to clear TIF_NOTIFY_SIGNAL and it can
> have more users.

I think it's fundamentaly wrong that we have several places and several
flags which handle task_work_run() instead of having exactly one place
and one flag.

Thanks,

        tglx