Re: [PATCH 3/3] task_work: use TIF_TASKWORK if available

From: Jens Axboe <axboe@kernel.dk>
To: Oleg Nesterov <oleg@redhat.com>
Cc: linux-kernel@vger.kernel.org, io-uring@vger.kernel.org,
	peterz@infradead.org, tglx@linutronix.de
Subject: Re: [PATCH 3/3] task_work: use TIF_TASKWORK if available
Date: Fri, 2 Oct 2020 09:53:47 -0600	[thread overview]
Message-ID: <fb15a44f-3fdf-2f12-ee85-f229bd261419@kernel.dk> (raw)
In-Reply-To: <20201002151415.GA29066@redhat.com>

On 10/2/20 9:14 AM, Oleg Nesterov wrote:
> Heh. To be honest I don't really like 1-2 ;)
> 
> Unfortunately, I do not see a better approach right now. Let me think
> until Monday, it is not that I think I will find a better solution, but
> I'd like to try anyway.
> 
> Let me comment 3/3 for now.

Thanks, appreciate your time on this!

>> +static void task_work_signal(struct task_struct *task)
>> +{
>> +#ifndef TIF_TASKWORK
>> +	unsigned long flags;
>> +
>> +	/*
>> +	 * Only grab the sighand lock if we don't already have some
>> +	 * task_work pending. This pairs with the smp_store_mb()
>> +	 * in get_signal(), see comment there.
>> +	 */
>> +	if (!(READ_ONCE(task->jobctl) & JOBCTL_TASK_WORK) &&
>> +	    lock_task_sighand(task, &flags)) {
>> +		task->jobctl |= JOBCTL_TASK_WORK;
>> +		signal_wake_up(task, 0);
>> +		unlock_task_sighand(task, &flags);
>> +	}
>> +#else
>> +	set_tsk_thread_flag(task, TIF_TASKWORK);
>> +	set_notify_resume(task);
>> +#endif
> 
> Again, I can't understand. task_work_signal(task) should set TIF_TASKWORK
> to make signal_pending() = T _and_ wake/kick the target up, just like
> signal_wake_up() does. Why do we set TIF_NOTIFY_RESUME ?
> 
> 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.
> 
> What do you think?

I like that. It'll achieve the same thing as far as I'm concerned, but not
tie the functionality to task_work. Not that we have anything that'd use
it right now, but it still seems like a better base.

I'll adapt patch 2+3 for this, thanks Oleg.

-- 
Jens Axboe