Re: [RFC v4] wait: prevent waiter starvation in __wait_on_bit_lock

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Jan 23, 2009 at 12:35:41PM +0100, Oleg Nesterov wrote:
> 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.

Agreed.  I will whip up another series that adds
finish_wait_exclusive() and adjusts the problematic callsites.

	Hannes