Re: [PATCH] task_work: Consume only item at a time while invoking the callbacks.

[Oleg Nesterov <oleg@redhat.com>]

Well... I won't really argue because I can't suggest a better fix at
least right now. Most probably never.

However, let me say that this patch doesn't make me happy ;) See below.

On 02/21, Sebastian Andrzej Siewior wrote:
>
> Oleg pointed out that this might be problematic if one closes 2.000.000
> files at once. While testing this scenario by opening that many files
> following by exit() to ensure that all files are closed at once, I did
> not observe anything outside of noise.

and this probably means that we can revert c82199061009 ("task_work: remove
fifo ordering guarantee") and restore the fifo ordering which IMO makes much
more sense.

But:

> Fixes: c5d93d23a2601 ("perf: Enqueue SIGTRAP always via task_work.")

Yes. So, to fix this specific problem in perf this patch changes task_work.c

And after this change we can never enforce a "clear" ordering, fifo or even lifo.
The ordering is simply "unpredictable/random".

I'll try to find and read the previous discussions tomorrow, but iirc Frederic
had another solution?

Oleg.

> --- a/kernel/task_work.c
> +++ b/kernel/task_work.c
> @@ -194,7 +194,7 @@ bool task_work_cancel(struct task_struct *task, struct callback_head *cb)
>  void task_work_run(void)
>  {
>  	struct task_struct *task = current;
> -	struct callback_head *work, *head, *next;
> +	struct callback_head *work, *head;
>
>  	for (;;) {
>  		/*
> @@ -202,17 +202,7 @@ void task_work_run(void)
>  		 * work_exited unless the list is empty.
>  		 */
>  		work = READ_ONCE(task->task_works);
> -		do {
> -			head = NULL;
> -			if (!work) {
> -				if (task->flags & PF_EXITING)
> -					head = &work_exited;
> -				else
> -					break;
> -			}
> -		} while (!try_cmpxchg(&task->task_works, &work, head));
> -
> -		if (!work)
> +		if (!work && !(task->flags & PF_EXITING))
>  			break;
>  		/*
>  		 * Synchronize with task_work_cancel_match(). It can not remove
> @@ -220,13 +210,24 @@ void task_work_run(void)
>  		 * But it can remove another entry from the ->next list.
>  		 */
>  		raw_spin_lock_irq(&task->pi_lock);
> +		do {
> +			head = NULL;
> +			if (work) {
> +				head = READ_ONCE(work->next);
> +			} else {
> +				if (task->flags & PF_EXITING)
> +					head = &work_exited;
> +				else
> +					break;
> +			}
> +		} while (!try_cmpxchg(&task->task_works, &work, head));
>  		raw_spin_unlock_irq(&task->pi_lock);
>
> -		do {
> -			next = work->next;
> -			work->func(work);
> -			work = next;
> +		if (!work)
> +			break;
> +		work->func(work);
> +
> +		if (head)
>  			cond_resched();
> -		} while (work);
>  	}
>  }
> --
> 2.47.2
>