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Date:	Fri, 27 Aug 2010 14:29:28 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Hugh Dickins <hughd@...gle.com>
cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: fix hang on anon_vma->root->lock

On Fri, 27 Aug 2010, Hugh Dickins wrote:

> Eh?  My solution was a second page_mapped(page) test i.e. testing an atomic.

Argh. Right. Looked like a global to me. Did not see the earlier local
def.

If you still use a pointer then what does insure that the root
pointer was not changed after the ACCESS_ONCE? The free semantics
of an anon_vma?

Since there is no lock taken before the mapped check none of the
earlier reads from the anon vma structure nor the page mapped check
necessarily reflect a single state of the anon_vma.




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