linux-kernel - Re: [patch 1/6] mmu

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Date:	Tue, 29 Jan 2008 15:34:17 +0100
From:	Andrea Arcangeli <andrea@...ranet.com>
To:	Christoph Lameter <clameter@....com>
Cc:	Robin Holt <holt@....com>, Avi Kivity <avi@...ranet.com>,
	Izik Eidus <izike@...ranet.com>, Nick Piggin <npiggin@...e.de>,
	kvm-devel@...ts.sourceforge.net,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, steiner@....com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	daniel.blueman@...drics.com, Hugh Dickins <hugh@...itas.com>
Subject: Re: [patch 1/6] mmu_notifier: Core code

On Tue, Jan 29, 2008 at 02:59:14PM +0100, Andrea Arcangeli wrote:
> The down_write is garbage. The caller should put it around
> mmu_notifier_register if something. The same way the caller should
> call synchronize_rcu after mmu_notifier_register if it needs
> synchronous behavior from the notifiers. The default version of
> mmu_notifier_register shouldn't be cluttered with unnecessary locking.

Ooops my spinlock was gone from the notifier head.... so the above
comment is wrong sorry! I thought down_write was needed to serialize
against some _external_ event, not to serialize the list updates in
place of my explicit lock. The critical section is so small that a
semaphore is the wrong locking choice, that's why I assumed it was for
an external event. Anyway RCU won't be optimal for a huge flood of
register/unregister, I agree the down_write shouldn't create much
contention and it saves 4 bytes from each mm_struct, and we can always
change it to a proper spinlock later if needed.
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