linux-kernel - Re: [PATCH 01 of 12] Core of mmu notifiers

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Message-ID: <20080422164615.GG24536@duo.random>
Date:	Tue, 22 Apr 2008 18:46:15 +0200
From:	Andrea Arcangeli <andrea@...ranet.com>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	Christoph Lameter <clameter@....com>,
	Nick Piggin <npiggin@...e.de>, Jack Steiner <steiner@....com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	kvm-devel@...ts.sourceforge.net,
	Kanoj Sarcar <kanojsarcar@...oo.com>,
	Roland Dreier <rdreier@...co.com>,
	Steve Wise <swise@...ngridcomputing.com>,
	linux-kernel@...r.kernel.org, Avi Kivity <avi@...ranet.com>,
	linux-mm@...ck.org, Robin Holt <holt@....com>,
	general@...ts.openfabrics.org, Hugh Dickins <hugh@...itas.com>,
	akpm@...ux-foundation.org, Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH 01 of 12] Core of mmu notifiers

On Tue, Apr 22, 2008 at 05:37:38PM +0200, Eric Dumazet wrote:
> I am saying your intent was probably to test
>
> else if ((unsigned long)*(spinlock_t **)a ==
> 	    (unsigned long)*(spinlock_t **)b)
> 		return 0;

Indeed...

> Hum, it's not a micro-optimization, but a bug fix. :)

The good thing is that even if this bug would lead to a system crash,
it would be still zero risk for everybody that isn't using KVM/GRU
actively with mmu notifiers. The important thing is that this patch
has zero risk to introduce regressions into the kernel, both when
enabled and disabled, it's like a new driver. I'll shortly resend 1/12
and likely 12/12 for theoretical correctness. For now you can go ahead
testing with this patch as it'll work fine despite of the bug (if it
wasn't the case I would have noticed already ;).
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