Re: 2.6.22 -mm merge plans -- vm bugfixes

[Nick Piggin]

Andrew Morton wrote:
> On Thu, 03 May 2007 11:32:23 +1000 Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:
>
>
> > void fastcall unlock_page(struct page *page)
> > {
> > +	VM_BUG_ON(!PageLocked(page));
> > 	smp_mb__before_clear_bit();
> > -	if (!TestClearPageLocked(page))
> > -		BUG();
> > -	smp_mb__after_clear_bit(); 
> > -	wake_up_page(page, PG_locked);
> > +	ClearPageLocked(page);
> > +	if (unlikely(test_bit(PG_waiters, &page->flags))) {
> > +		clear_bit(PG_waiters, &page->flags);
> > +		wake_up_page(page, PG_locked);
> > +	}
> > }
>
>
> Why is that significantly faster than plain old wake_up_page(), which
> tests waitqueue_active()?

Because it needs fewer barriers and doesn't touch random a random hash
cacheline in the fastpath.

--
SUSE Labs, Novell Inc.
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