From: Rik van Riel <riel@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Mel Gorman <mel@csn.ul.ie>,
Andrea Arcangeli <aarcange@redhat.com>,
Minchan Kim <minchan.kim@gmail.com>,
Linux-MM <linux-mm@kvack.org>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 4/5] always lock the root (oldest) anon_vma
Date: Wed, 12 May 2010 18:18:11 -0400 [thread overview]
Message-ID: <4BEB2923.8030200@redhat.com> (raw)
In-Reply-To: <alpine.LFD.2.00.1005121441350.3711@i5.linux-foundation.org>
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?
--
All rights reversed
WARNING: multiple messages have this Message-ID (diff)
From: Rik van Riel <riel@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Mel Gorman <mel@csn.ul.ie>,
Andrea Arcangeli <aarcange@redhat.com>,
Minchan Kim <minchan.kim@gmail.com>,
Linux-MM <linux-mm@kvack.org>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 4/5] always lock the root (oldest) anon_vma
Date: Wed, 12 May 2010 18:18:11 -0400 [thread overview]
Message-ID: <4BEB2923.8030200@redhat.com> (raw)
In-Reply-To: <alpine.LFD.2.00.1005121441350.3711@i5.linux-foundation.org>
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?
--
All rights reversed
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next prev parent reply other threads:[~2010-05-12 22:18 UTC|newest]
Thread overview: 106+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-05-12 17:38 [PATCH 0/5] always lock the root anon_vma Rik van Riel
2010-05-12 17:38 ` Rik van Riel
2010-05-12 17:39 ` [PATCH 1/5] rename anon_vma_lock to vma_lock_anon_vma Rik van Riel
2010-05-12 17:39 ` Rik van Riel
2010-05-12 20:57 ` Mel Gorman
2010-05-12 20:57 ` Mel Gorman
2010-05-13 0:30 ` KAMEZAWA Hiroyuki
2010-05-13 0:30 ` KAMEZAWA Hiroyuki
2010-05-12 17:39 ` [PATCH 3/5] track the root (oldest) anon_vma Rik van Riel
2010-05-12 17:39 ` Rik van Riel
2010-05-12 20:59 ` Mel Gorman
2010-05-12 20:59 ` Mel Gorman
2010-05-12 21:01 ` Rik van Riel
2010-05-12 21:01 ` Rik van Riel
2010-05-13 0:38 ` KAMEZAWA Hiroyuki
2010-05-13 0:38 ` KAMEZAWA Hiroyuki
2010-05-13 2:25 ` Rik van Riel
2010-05-13 2:25 ` Rik van Riel
2010-05-14 0:04 ` KAMEZAWA Hiroyuki
2010-05-14 0:04 ` KAMEZAWA Hiroyuki
2010-05-12 17:40 ` [PATCH 4/5] always lock " Rik van Riel
2010-05-12 17:40 ` Rik van Riel
2010-05-12 21:02 ` Mel Gorman
2010-05-12 21:02 ` Mel Gorman
2010-05-12 21:08 ` Rik van Riel
2010-05-12 21:08 ` Rik van Riel
2010-05-13 9:54 ` Mel Gorman
2010-05-13 9:54 ` Mel Gorman
2010-05-13 14:33 ` [PATCH -v2 " Rik van Riel
2010-05-13 14:33 ` Rik van Riel
2010-05-13 21:09 ` Andrew Morton
2010-05-13 21:09 ` Andrew Morton
2010-05-13 22:50 ` Rik van Riel
2010-05-13 22:50 ` Rik van Riel
2010-05-14 9:33 ` Mel Gorman
2010-05-14 9:33 ` Mel Gorman
2010-05-26 4:00 ` Rik van Riel
2010-05-26 4:00 ` Rik van Riel
2010-05-26 4:15 ` Andrew Morton
2010-05-26 4:15 ` Andrew Morton
2010-05-26 5:46 ` james toy
2010-05-26 5:46 ` james toy
2010-06-01 0:57 ` james toy
2010-06-01 0:57 ` james toy
2010-05-26 15:24 ` [PATCH -v2 0/5] always lock the root anon_vma Rik van Riel
2010-05-26 15:24 ` Rik van Riel
2010-05-26 15:25 ` [PATCH 1/5] rename anon_vma_lock to vma_lock_anon_vma Rik van Riel
2010-05-26 15:25 ` Rik van Riel
2010-05-26 17:25 ` Linus Torvalds
2010-05-26 17:25 ` Linus Torvalds
2010-05-26 19:01 ` Rik van Riel
2010-05-26 19:01 ` Rik van Riel
2010-05-26 19:25 ` Linus Torvalds
2010-05-26 19:25 ` Linus Torvalds
2010-05-26 19:35 ` Rik van Riel
2010-05-26 19:35 ` Rik van Riel
2010-05-26 15:25 ` [PATCH 2/5] change direct call of spin_lock(anon_vma->lock) to inline function Rik van Riel
2010-05-26 15:25 ` Rik van Riel
2010-05-26 15:26 ` [PATCH 3/5] track the root (oldest) anon_vma Rik van Riel
2010-05-26 15:26 ` Rik van Riel
2010-05-26 15:27 ` [PATCH 4/5] always lock " Rik van Riel
2010-05-26 15:27 ` Rik van Riel
2010-05-26 15:27 ` [PATCH 5/5] extend KSM refcounts to the anon_vma root Rik van Riel
2010-05-26 15:27 ` Rik van Riel
2010-05-12 21:55 ` [PATCH 4/5] always lock the root (oldest) anon_vma Linus Torvalds
2010-05-12 21:55 ` Linus Torvalds
2010-05-12 22:18 ` Rik van Riel [this message]
2010-05-12 22:18 ` Rik van Riel
2010-05-12 22:26 ` Linus Torvalds
2010-05-12 22:26 ` Linus Torvalds
2010-05-12 17:41 ` [PATCH 5/5] extend KSM refcounts to the anon_vma root Rik van Riel
2010-05-12 17:41 ` Rik van Riel
2010-05-12 21:07 ` Mel Gorman
2010-05-12 21:07 ` Mel Gorman
2010-05-12 21:09 ` Rik van Riel
2010-05-12 21:09 ` Rik van Riel
2010-05-13 11:26 ` Mel Gorman
2010-05-13 11:26 ` Mel Gorman
2010-05-13 13:11 ` Rik van Riel
2010-05-13 13:11 ` Rik van Riel
2010-05-13 13:24 ` Mel Gorman
2010-05-13 13:24 ` Mel Gorman
2010-05-13 14:34 ` [PATCH -v2 " Rik van Riel
2010-05-13 14:34 ` Rik van Riel
2010-05-19 1:05 ` Andrea Arcangeli
2010-05-19 1:05 ` Andrea Arcangeli
2010-05-12 17:41 ` [PATCH 2/5] change direct call of spin_lock(anon_vma->lock) to inline function Rik van Riel
2010-05-12 17:41 ` Rik van Riel
2010-05-12 20:58 ` Mel Gorman
2010-05-12 20:58 ` Mel Gorman
2010-05-13 0:32 ` KAMEZAWA Hiroyuki
2010-05-13 0:32 ` KAMEZAWA Hiroyuki
2010-05-20 22:42 ` [PATCH 6/5] adjust mm_take_all_locks to anon-vma-root locking Andrea Arcangeli
2010-05-20 22:42 ` Andrea Arcangeli
2010-05-20 23:07 ` Rik van Riel
2010-05-20 23:07 ` Rik van Riel
-- strict thread matches above, loose matches on Subject: below --
2010-05-26 19:38 [PATCH -v3 0/5] always lock the root anon_vma Rik van Riel
2010-05-26 19:40 ` [PATCH 4/5] always lock the root (oldest) anon_vma Rik van Riel
2010-05-26 19:40 ` Rik van Riel
2010-05-26 20:36 ` Larry Woodman
2010-05-26 20:36 ` Larry Woodman
2010-05-27 0:57 ` KAMEZAWA Hiroyuki
2010-05-27 0:57 ` KAMEZAWA Hiroyuki
2010-05-27 13:55 ` Minchan Kim
2010-05-27 13:55 ` Minchan Kim
2010-05-27 17:48 ` Mel Gorman
2010-05-27 17:48 ` Mel Gorman
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