linux-kernel - Re: [PATCH 1 of 9] Lock the entire mm to prevent any mmu related operation to happen

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Message-ID: <20080426005726.GA9514@duo.random>
Date:	Sat, 26 Apr 2008 02:57:26 +0200
From:	Andrea Arcangeli <andrea@...ranet.com>
To:	Robin Holt <holt@....com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Christoph Lameter <clameter@....com>,
	akpm@...ux-foundation.org, Nick Piggin <npiggin@...e.de>,
	Steve Wise <swise@...ngridcomputing.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, linux-mm@...ck.org,
	Kanoj Sarcar <kanojsarcar@...oo.com>,
	Roland Dreier <rdreier@...co.com>,
	Jack Steiner <steiner@....com>, linux-kernel@...r.kernel.org,
	Avi Kivity <avi@...ranet.com>, kvm-devel@...ts.sourceforge.net,
	general@...ts.openfabrics.org, Hugh Dickins <hugh@...itas.com>
Subject: Re: [PATCH 1 of 9] Lock the entire mm to prevent any mmu related
	operation to happen

On Fri, Apr 25, 2008 at 02:25:32PM -0500, Robin Holt wrote:
> I think you still need mm_lock (unless I miss something).  What happens
> when one callout is scanning mmu_notifier_invalidate_range_start() and
> you unlink.  That list next pointer with LIST_POISON1 which is a really
> bad address for the processor to track.

Ok, _release list_del_init qcan't race with that because it happens in
exit_mmap when no other mmu notifier can trigger anymore.

_unregister can run concurrently but it does list_del_rcu, that only
overwrites the pprev pointer with LIST_POISON2. The
mmu_notifier_invalidate_range_start won't crash on LIST_POISON1 thanks
to srcu.

Actually I did more changes than necessary, for example I noticed the
mmu_notifier_register can return a list_add_head instead of
list_add_head_rcu. _register can't race against _release thanks to the
mm_users temporary or implicit pin. _register can't race against
_unregister thanks to the mmu_notifier_mm->lock. And register can't
race against all other mmu notifiers thanks to the mm_lock.

At this time I've no other pending patches on top of v14-pre3 other
than the below micro-optimizing cleanup. It'd be great to have
confirmation that v14-pre3 passes GRU/XPMEM regressions tests as well
as my KVM testing already passed successfully on it. I'll forward
v14-pre3 mmu-notifier-core plus the below to Andrew tomorrow, I'm
trying to be optimistic here! ;)

diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
--- a/mm/mmu_notifier.c
+++ b/mm/mmu_notifier.c
@@ -187,7 +187,7 @@ int mmu_notifier_register(struct mmu_not
 	 * current->mm or explicitly with get_task_mm() or similar).
 	 */
 	spin_lock(&mm->mmu_notifier_mm->lock);
-	hlist_add_head_rcu(&mn->hlist, &mm->mmu_notifier_mm->list);
+	hlist_add_head(&mn->hlist, &mm->mmu_notifier_mm->list);
 	spin_unlock(&mm->mmu_notifier_mm->lock);
 out_unlock:
 	mm_unlock(mm, &data);
--
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