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Date: Tue, 22 Apr 2008 18:46:15 +0200
From: Andrea Arcangeli <andrea@qumranet.com>
To: Eric Dumazet <dada1@cosmosbay.com>
Cc: Christoph Lameter <clameter@sgi.com>, Nick Piggin <npiggin@suse.de>,
       Jack Steiner <steiner@sgi.com>, Peter Zijlstra <a.p.zijlstra@chello.nl>,
       kvm-devel@lists.sourceforge.net, Kanoj Sarcar <kanojsarcar@yahoo.com>,
       Roland Dreier <rdreier@cisco.com>,
       Steve Wise <swise@opengridcomputing.com>, linux-kernel@vger.kernel.org,
       Avi Kivity <avi@qumranet.com>, linux-mm@kvack.org,
       Robin Holt <holt@sgi.com>, general@lists.openfabrics.org,
       Hugh Dickins <hugh@veritas.com>, akpm@linux-foundation.org,
       Rusty Russell <rusty@rustcorp.com.au>
Subject: Re: [PATCH 01 of 12] Core of mmu notifiers
Message-ID: <20080422164615.GG24536@duo.random>
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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 ;).