Re: [PATCH V2] task_work: avoid unneeded cmpxchg() in task_work_run()

[Oleg Nesterov <oleg@redhat.com>]

On 10/10, Lai Jiangshan wrote:
>
> --- a/kernel/task_work.c
> +++ b/kernel/task_work.c
> @@ -52,16 +52,7 @@ void task_work_run(void)
>  	struct callback_head *work, *head, *next;
>
>  	for (;;) {
> -		/*
> -		 * work->func() can do task_work_add(), do not set
> -		 * work_exited unless the list is empty.
> -		 */
> -		do {
> -			work = ACCESS_ONCE(task->task_works);
> -			head = !work && (task->flags & PF_EXITING) ?
> -				&work_exited : NULL;
> -		} while (cmpxchg(&task->task_works, work, head) != work);
> -
> +		work = xchg(&task->task_works, NULL);
>  		if (!work)
>  			break;
>  		/*
> @@ -90,3 +81,17 @@ void task_work_run(void)
>  		} while (work);
>  	}
>  }
> +
> +void exit_task_work(struct task_struct *task)
> +{
> +	for (;;) {
> +		/*
> +		 * work->func() can do task_work_add(), do not set
> +		 * work_exited unless the list is empty.
> +		 */
> +		if (unlikely(task->task_works))
> +			task_work_run();
> +		if (cmpxchg(&task->task_works, NULL, &work_exited) == NULL)
> +			break;
> +	}
> +}

I agree, this looks fine.

But if you add "unlikely" before task_work_run(), then probably
it should do

	while (cmpxchg(&task->task_works, NULL, work_exited))
		task_work_run();

? it looks more simple/clean.

(OTOH I am not sure "unlikely" is true, note that exit_files() will
 offload ____fput() to task_work_run()).

But you did not answer, and I am curious. What was your original
motivation? Is xchg really faster than cmpxchg?

Oleg.