Re: [PATCH] task_work_run: don't take ->pi_lock unconditionally

[Oleg Nesterov <oleg@redhat.com>]

On 02/20, Peter Zijlstra wrote:
>
> On Thu, Feb 20, 2020 at 06:22:02PM +0100, Oleg Nesterov wrote:
> > @@ -68,10 +65,10 @@ task_work_cancel(struct task_struct *task, task_work_func_t func)
> >  	 * we raced with task_work_run(), *pprev == NULL/exited.
> >  	 */
> >  	raw_spin_lock_irqsave(&task->pi_lock, flags);
> > +	for (work = READ_ONCE(*pprev); work; ) {
> >  		if (work->func != func)
> >  			pprev = &work->next;
>
> But didn't you loose the READ_ONCE() of *pprev in this branch?

Argh, yes ;)

> > @@ -97,16 +94,16 @@ void task_work_run(void)
> >  		 * work->func() can do task_work_add(), do not set
> >  		 * 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)
> >  			break;
>
> But given that, as you say, cancel() could have gone and stole our head,
> should we not try and install &work_exiting when PF_EXITING in that
> case?

cancel() can't do this, as long as we use cmpxchg/try_cmpxchg, not xchg().
This is what the comment before lock/unlock below tries to explain.

> That is; should we not do continue in that case, instead of break.

This is what we should do if we use xchg() like your previous version did.
Or I am totally confused. Hmm, and when I re-read my words it looks as if
I am trying to confuse you.

So lets "simplify" this code assuming that PF_EXITING is set:

		work = READ_ONCE(task->task_works);
		do {
			head = NULL;
			if (!work)
				head = &work_exited;
		} while (!try_cmpxchg(&task->task_works, &work, head));

		if (!work)
			break;

If work == NULL after try_cmpxchg() _succeeds_, then the new "head" must
be work_exited and we have nothing to do.

If it was nullified by try_cmpxchg(&work) because we raced with cancel_(),
then this try_cmpxchg() should have been failed.

Right?

> @@ -69,9 +68,12 @@ task_work_cancel(struct task_struct *tas
>  	 */
>  	raw_spin_lock_irqsave(&task->pi_lock, flags);
>  	while ((work = READ_ONCE(*pprev))) {
> -		if (work->func != func)
> +		if (work->func != func) {
>  			pprev = &work->next;
> -		else if (cmpxchg(pprev, work, work->next) == work)
> +			continue;
> +		}
> +
> +		if (try_cmpxchg(pprev, &work, work->next))
>  			break;

perhaps I misread this code, but it looks a bit strange to me... it doesn't
differ from

	while ((work = READ_ONCE(*pprev))) {
		if (work->func != func)
			pprev = &work->next;
		else if (try_cmpxchg(pprev, &work, work->next))
			break;
	}

either way it is correct, the only problem is that we do not need (want)
another READ_ONCE() if try_cmpxchg() fails.

>  void task_work_run(void)
>  {
>  	struct task_struct *task = current;
> -	struct callback_head *work, *head, *next;
> +	struct callback_head *work, *next;
>  
>  	for (;;) {
> -		/*
> -		 * work->func() can do task_work_add(), do not set
> -		 * work_exited unless the list is empty.
> -		 */
> -		do {
> -			head = NULL;
> -			work = READ_ONCE(task->task_works);
> -			if (!work) {
> -				if (task->flags & PF_EXITING)
> -					head = &work_exited;
> -				else
> -					break;
> -			}
> -		} while (cmpxchg(&task->task_works, work, head) != work);
> +		work = READ_ONCE(task->task_works);
> +		if (!work) {
> +			if (!(task->flags & PF_EXITING))
> +				return;
> +
> +			/*
> +			 * work->func() can do task_work_add(), do not set
> +			 * work_exited unless the list is empty.
> +			 */
> +			if (try_cmpxchg(&task->task_works, &work, &work_exited))
> +				return;
> +		}
> +
> +		work = xchg(&task->task_works, NULL);
> +		if (!work)
> +			continue;

looks correct...

Oleg.