Re: [RESEND PATCH] fs/pipe: Introduce a check to skip sleeping processes during pipe read/write

[Oleg Nesterov <oleg@redhat.com>]

On 12/28, Oleg Nesterov wrote:
>
> >  int __wake_up(struct wait_queue_head *wq_head, unsigned int mode,
> >  	      int nr_exclusive, void *key)
> >  {
> > +	if (list_empty(&wq_head->head)) {
> > +		struct list_head *pn;
> > +
> > +		/*
> > +		 * pairs with spin_unlock_irqrestore(&wq_head->lock);
> > +		 * We actually do not need to acquire wq_head->lock, we just
> > +		 * need to be sure that there is no prepare_to_wait() that
> > +		 * completed on any CPU before __wake_up was called.
> > +		 * Thus instead of load_acquiring the spinlock and dropping
> > +		 * it again, we load_acquire the next list entry and check
> > +		 * that the list is not empty.
> > +		 */
> > +		pn = smp_load_acquire(&wq_head->head.next);
> > +
> > +		if(pn == &wq_head->head)
> > +			return 0;
> > +	}
>
> Too subtle for me ;)
>
> I have some concerns, but I need to think a bit more to (try to) actually
> understand this change.

If nothing else, consider

	int CONDITION;
	wait_queue_head_t WQ;

	void wake(void)
	{
		CONDITION = 1;
		wake_up(WQ);
	}

	void wait(void)
	{
		DEFINE_WAIT_FUNC(entry, woken_wake_function);

		add_wait_queue(WQ, entry);
		if (!CONDITION)
			wait_woken(entry, ...);
		remove_wait_queue(WQ, entry);
	}

this code is correct even if LOAD(CONDITION) can leak into the critical
section in add_wait_queue(), so CPU running wait() can actually do

		// add_wait_queue
		spin_lock(WQ->lock);
		LOAD(CONDITION);	// false!
		list_add(entry, head);
		spin_unlock(WQ->lock);

		if (!false)		// result of the LOAD above
			wait_woken(entry, ...);

Now suppose that another CPU executes wake() between LOAD(CONDITION)
and list_add(entry, head). With your patch wait() will miss the event.
The same for __pollwait(), I think...

No?

Oleg.