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

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Date:   Fri, 2 Oct 2020 10:42:46 -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 9:52 AM, Jens Axboe wrote:
> 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.

I re-spun (and re-tested) the series, now based on TIF_NOTIFY_SIGNAL
instead. I won't be sending this one out before we've discussed it
some more, but wanted to let you know what it currently looks like:

https://git.kernel.dk/cgit/linux-block/log/?h=tif-task_work

-- 
Jens Axboe