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

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Message-ID: <Pine.LNX.4.64.0801311748270.24297@schroedinger.engr.sgi.com>
Date:	Thu, 31 Jan 2008 17:52:06 -0800 (PST)
From:	Christoph Lameter <clameter@....com>
To:	Andrea Arcangeli <andrea@...ranet.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 Fri, 1 Feb 2008, Andrea Arcangeli wrote:

> 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.

Why would the list_del generate problems?

> 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?).

You could pin via mm_users? Then it would be entirely safe and no need for 
rcu tricks?

OTOH if there are mm_count users like in KVM: Could we guarantee that 
they do not perform any operations with the mmu notifier list? Then we 
would be safe as well.

> 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.

It is useful if a device driver has a list of data segments that contain 
struct mmu_notifiers. The device driver can inspect the mmu_notifier and 
reliably conclude that the beast is inactive.
 
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