Re: [ISSUE] The time cost of IOSQE_IO_LINK

Peter Zijlstra
2020-02-18T15:08:02Z

On Tue, Feb 18, 2020 at 03:56:46PM +0100, Oleg Nesterov wrote:
> On 02/18, Peter Zijlstra wrote:
> >
> > But this has me wondering about task_work_run(), as it is it will
> > unconditionally take pi_lock,
>
> because spin_unlock_wait() was removed ;) task_work_run() doesn't
> really need to take pi_lock at all.

Right.

> > --- a/kernel/task_work.c
> > +++ b/kernel/task_work.c
> > @@ -93,16 +93,20 @@ void task_work_run(void)
> > struct callback_head *work, *head, *next;
> >
> > for (;;) {
> > + work = READ_ONCE(task->task_work);
> > + if (!work)
> > + break
>
> This is wrong if PF_EXITING is set, in this case we must set
> task->task_works = work_exited.

Indeed!

> > +
> > /*
> > * work->func() can do task_work_add(), do not set
> > * work_exited unless the list is empty.
> > */
> > raw_spin_lock_irq(&task->pi_lock);
> > do {
> > - work = READ_ONCE(task->task_works);
> > - head = !work && (task->flags & PF_EXITING) ?
> > - &work_exited : NULL;
> > - } while (cmpxchg(&task->task_works, work, head) != work);
> > + head = NULL;
> > + if (unlikely(!work && (task->flags & PF_EXITING)))
> > + head = &work_exited;
> > + } while (!try_cmpxchg(&task->task_works, &work, head));
> > raw_spin_unlock_irq(&task->pi_lock);
> >
> > if (!work)
>
> otherwise I think this is correct, but how about the patch below?
> Then this code can be changed to use try_cmpxchg().

Works for me. Thanks!