linux-kernel - Re: [PATCH 08 of 11] anon-vma-rwsem

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Message-ID: <alpine.LFD.1.10.0805071655100.3024@woody.linux-foundation.org>
Date:	Wed, 7 May 2008 17:03:30 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Christoph Lameter <clameter@....com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <andrea@...ranet.com>, steiner@....com,
	holt@....com, npiggin@...e.de, a.p.zijlstra@...llo.nl,
	kvm-devel@...ts.sourceforge.net, kanojsarcar@...oo.com,
	rdreier@...co.com, swise@...ngridcomputing.com,
	linux-kernel@...r.kernel.org, avi@...ranet.com, linux-mm@...ck.org,
	general@...ts.openfabrics.org, hugh@...itas.com,
	rusty@...tcorp.com.au, aliguori@...ibm.com, chrisw@...hat.com,
	marcelo@...ck.org, dada1@...mosbay.com, paulmck@...ibm.com
Subject: Re: [PATCH 08 of 11] anon-vma-rwsem



On Wed, 7 May 2008, Christoph Lameter wrote:
> 
> Multiple vmas may share the same mapping or refer to the same anonymous 
> vma. The above code will deadlock since we may take some locks multiple 
> times.

Ok, so that actually _is_ a problem. It would be easy enough to also add 
just a flag to the vma (VM_MULTILOCKED), which is still cleaner than doing 
a vmalloc and a whole sort thing, but if this is really rare, maybe Ben's 
suggestion of just using stop-machine is actually the right one just 
because it's _so_ simple.

(That said, we're not running out of vm flags yet, and if we were, we 
could just add another word. We're already wasting that space right now on 
64-bit by calling it "unsigned long").

		Linus
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