linux-kernel - Re: [PATCH 3/4] hugetlb, rmap: fix confusing page locking in hugetlb

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Date:	Fri, 10 Sep 2010 10:15:46 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Christoph Lameter <cl@...ux.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <andi@...stfloor.org>, linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH 3/4] hugetlb, rmap: fix confusing page locking in
 hugetlb_cow()



On Fri, 10 Sep 2010, Naoya Horiguchi wrote:
>  
> -	if (!pagecache_page) {
> -		page = pte_page(entry);
> +	/*
> +	 * hugetlb_cow() requires page locks of pte_page(entry) and
> +	 * pagecache_page, so here we need take the former one
> +	 * when page != pagecache_page or !pagecache_page.
> +	 */
> +	page = pte_page(entry);
> +	if (page != pagecache_page)
>  		lock_page(page);

Why isn't this a potential deadlock? You have two pages, and lock them 
both. Is there some ordering guarantee that says that 'pagecache_page' and 
'page' will always be in a certain relationship so that you cannot get 
A->B and B->A lock ordering?

Please document that ordering rule if so.

			Linus
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