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Date: Thu, 3 May 2007 09:52:24 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Hugh Dickins <hugh@veritas.com>, linux-kernel@vger.kernel.org,
       linux-mm@kvack.org, Andrea Arcangeli <andrea@suse.de>,
       Christoph Hellwig <hch@infradead.org>
Subject: Re: 2.6.22 -mm merge plans -- vm bugfixes
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On Thu, 03 May 2007 11:32:23 +1000 Nick Piggin <nickpiggin@yahoo.com.au> 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()?