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

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Date:   Fri, 2 Oct 2020 09:52:24 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Oleg Nesterov <oleg@...hat.com>
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 10/2/20 9:31 AM, Thomas Gleixner wrote:
> 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.

I don't disagree with that. I know it's not happening in this series, but
if we to the TIF_NOTIFY_SIGNAL route and get all archs supporting that,
then we can kill the signal and notify resume part of running task_work.
And that leaves us with exactly one place that runs it.

So we can potentially improve the current situation in that regard.

-- 
Jens Axboe