linux-kernel - Re: 2.6.22 -mm merge plans -- vm bugfixes

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Date:	Fri, 04 May 2007 14:16:01 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Hugh Dickins <hugh@...itas.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Andrea Arcangeli <andrea@...e.de>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: 2.6.22 -mm merge plans -- vm bugfixes

Andrew Morton wrote:
> On Thu, 03 May 2007 11:32:23 +1000 Nick Piggin <nickpiggin@...oo.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()?

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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