linux-kernel - Re: [PATCH 4/5] always lock the root (oldest) anon

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Date:	Wed, 12 May 2010 18:18:11 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Linux-MM <linux-mm@...ck.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/5] always lock the root (oldest) anon_vma

On 05/12/2010 05:55 PM, Linus Torvalds wrote:

> Wouldn't it be sufficient to do
>
> 	if (atomic_dec_and_test(&anon_vma->ksm_refcount)) {
> 		anon_vma_lock(anon_vma);
>
> instead? The "atomic_dec_and_lock()" semantics are _much_ stricter than a
> regular "decrement and test and then lock", and that strictness means that
> it's way more complicated and expensive. So if you don't need the
> semantics, you shouldn't use them.

I suspect the atomic_dec_and_lock in the KVM code is being used
to prevent the following race:

1) KSM code reduces the refcount to 0

2)                               munmap on other CPU frees the anon_vma

3) KSM code takes the anon_vma lock,
    which now lives in freed memory

Am I totally confused by this and can we use a nicer approach?

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