linux-kernel - Re: [PATCH] 3/4 combine RCU with seqlock to allow mmu notifier methods to sleep (#v9 was 1/4)

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Message-ID: <Pine.LNX.4.64.0803071207540.6815@schroedinger.engr.sgi.com>
Date:	Fri, 7 Mar 2008 12:10:54 -0800 (PST)
From:	Christoph Lameter <clameter@....com>
To:	Andrea Arcangeli <andrea@...ranet.com>
cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Jack Steiner <steiner@....com>, Nick Piggin <npiggin@...e.de>,
	akpm@...ux-foundation.org, Robin Holt <holt@....com>,
	Avi Kivity <avi@...ranet.com>, kvm-devel@...ts.sourceforge.net,
	general@...ts.openfabrics.org,
	Steve Wise <swise@...ngridcomputing.com>,
	Roland Dreier <rdreier@...co.com>,
	Kanoj Sarcar <kanojsarcar@...oo.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	daniel.blueman@...drics.com
Subject: Re: [PATCH] 3/4 combine RCU with seqlock to allow mmu notifier
 methods to sleep (#v9 was 1/4)

On Fri, 7 Mar 2008, Andrea Arcangeli wrote:

> In the meantime I've also been thinking that we could need the
> write_seqlock in mmu_notifier_register, to know when to restart the
> loop if somebody does a mmu_notifier_register;
> synchronize_rcu(). Otherwise there's no way to be sure the mmu
> notifier will start firing immediately after synchronize_rcu. I'm
> unsure if it's acceptable that in-progress mmu notifier invocations,
> don't need to notice the fact that somebody did mmu_notifier_register;
> synchronize_rcu. If they don't need to notice, then we can just drop
> unregister and all rcu_read_lock()s instead of adding write_seqlock to
> the register operation.

This is all getting into some very complicated issues.....

> Overall my effort is to try to avoid expand the list walk with
> explicit memory barriers like in EMM while trying to be equally
> efficient.

The smp_rmb is such a big problem? You have seqlock, rcu etc all in there 
as well. I doubt that this is more efficient.

> Another issue is that the _begin/_end logic doesn't provide any
> guarantee that the _begin will start firing before _end, if a kernel
> module is loaded while another cpu is already running inside some
> munmap operation etc.. The KVM usage of mmu notifier has no problem
> with that detail, but KVM doesn't use _begin at all, I wonder if
> others would have problems. This is a kind of a separate problem, but
> quite related to the question if the notifiers must be guaranteed to
> start firing immediately after mmu_notifier_unregister;synchronize_rcu
> or not, that's why I mentioned it here.

Ahh. Yes that is an interesting issue. If a device driver cannot handle 
this then _begin must prohibit module loading. That means not allowing 
stop_machine_run I guess which should not be that difficult.
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