linux-kernel - Re: mmu_notifier: Move mmu_notifier_release up to get rid of the invalidat

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Date:	Fri, 1 Feb 2008 01:13:55 +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>,
	kvm-devel@...ts.sourceforge.net,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, steiner@....com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	daniel.blueman@...drics.com
Subject: Re: mmu_notifier: Move mmu_notifier_release up to get rid of the
	invalidat_all() callback

On Thu, Jan 31, 2008 at 02:21:58PM -0800, Christoph Lameter wrote:
> Is this okay for KVM too?

->release isn't implemented at all in KVM, only the list_del generates
complications.

I think current code could be already safe through the mm_count pin,
becasue KVM relies on the fact anybody pinning through mm_count like
KVM does, is forbidden to call unregister and it's forced to wait the
auto-disarming when mm_users hits zero, but I feel like something's
still wrong if I think that I'm not using call_rcu to free the
notifier (OTOH we agreed the list had to be frozen and w/o readers
(modulo _release) before _release is called, so if this initial
assumption is ok it seems I may be safe w/o call_rcu?).

But it's really tricky path. Anyway this is the last of my worries
right now, it works perfectly fine with a single user obviously, and
the moment KVM threads runs remotely through GRU/XPMEM isn't happening
too soon ;) so let's concentrate on the rest first. I can say
hlist_del_init doesn't seem to provide any benefit given nobody could
possibly decide to call register or unregister after _release run.
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