lkml.org 
[lkml]   [2009]   [Jan]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [RFC v4] wait: prevent waiter starvation in __wait_on_bit_lock
On 01/23, Johannes Weiner wrote:
>
> On Thu, Jan 22, 2009 at 09:25:50PM +0100, Oleg Nesterov wrote:
> > On 01/21, Johannes Weiner wrote:
> > >
> > int finish_wait_xxx(wait_queue_head_t *q, wait_queue_t *wait)
> > {
> > unsigned long flags;
> > int woken;
> >
> > __set_current_state(TASK_RUNNING);
> > spin_lock_irqsave(&q->lock, flags);
> > woken = list_empty(&wait->task_list);
> > list_del_init(&wait->task_list);
> > spin_unlock_irqrestore(&q->lock, flags);
> >
> > return woken;
> > }
>
> Hehe, there is only n solutions to this problem. I had thought about
> that too, even written it down. But I was not sure if taking the
> spinlock, toggling irqs and (re)storing the flags is better than an
> untaken branch. ;)

Yes. Fortunately, this is "unlikely" path.

> > if (test_bit(q->key.bit_nr, q->key.flags)) {
> > if ((ret = (*action)(q->key.flags))) {
> > if (finish_wait_xxx(...))
> > __wake_up_bit(...);
> > return ret;
> > }
> > }
>
> If you don't mind putting a second finish_wait() in there (you still
> need the one after the loop, right?), we can fix up my version to not
> check ret twice but do finish_wait() as you describe and then the
> test_bit() && wake up:
>
> do {
> if (test_bit())
> if ((ret = action())) {
> finish_wait()
> smp_rmb()
> if (!test_bit())
> __wake_up_bit()

Yes sure. Except this wakeup can be false.

> > int finish_wait_yyy(wait_queue_head_t *q, wait_queue_t *wait,
> > int mode, void *key)
> > {
> > unsigned long flags;
> > int woken;
> >
> > __set_current_state(TASK_RUNNING);
> > spin_lock_irqsave(&q->lock, flags);
> > woken = list_empty(&wait->task_list);
> > if (woken)
> > __wake_up_common(q, mode, 1, key);
> > else
> > list_del_init(&wait->task_list);
> > spin_unlock_irqrestore(&q->lock, flags);
> >
> > return woken;
> > }
> >
> > Perhaps a bit too much for this particular case, but I am thinking
> > about other cases when we need to abort the exclusive wait.
> >
> > For example, don't we have the similar problems with
> > wait_event_interruptible_exclusive() ?
>
> Yeah, we do IIUC. Then having finish_wait() extended is probably a
> good idea.

Yes.

It is no that I think this new helper is really needed for this
particular case, personally I agree with the patch you sent.

But if we have other places with the similar problem, then perhaps
it is better to introduce the special finish_wait_exclusive() or
whatever.

Oleg.



\
 
 \ /
  Last update: 2009-01-23 12:41    [W:1.347 / U:0.040 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site