From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jack Steiner <steiner-sJ/iWh9BUns@public.gmane.org>
Subject: Re: [patch 1/6] mmu_notifier: Core code
Date: Wed, 30 Jan 2008 09:53:06 -0600
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Cc: Nick Piggin <npiggin-l3A5Bk7waGM@public.gmane.org>,
	Peter Zijlstra <a.p.zijlstra-/NLkJaSkS4VmR6Xm/wNWPw@public.gmane.org>, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org,
	Benjamin Herrenschmidt <benh-XVmvHMARGAS8U2dJNN8I7kB+6BGkLq7r@public.gmane.org>,
	linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>,
	kvm-devel-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org, daniel.blueman-xqY44rlHlBpWk0Htik3J/w@public.gmane.org,
	Robin Holt <holt-sJ/iWh9BUns@public.gmane.org>, Hugh Dickins <hugh-DTz5qymZ9yRBDgjK7y7TUQ@public.gmane.org>,
	Christoph Lameter <clameter-sJ/iWh9BUns@public.gmane.org>
To: Andrea Arcangeli <andrea-atKUWr5tajBWk0Htik3J/w@public.gmane.org>
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On Wed, Jan 30, 2008 at 04:37:49PM +0100, Andrea Arcangeli wrote:
> On Tue, Jan 29, 2008 at 06:29:10PM -0800, Christoph Lameter wrote:
> > +void mmu_notifier_release(struct mm_struct *mm)
> > +{
> > +	struct mmu_notifier *mn;
> > +	struct hlist_node *n, *t;
> > +
> > +	if (unlikely(!hlist_empty(&mm->mmu_notifier.head))) {
> > +		rcu_read_lock();
> > +		hlist_for_each_entry_safe_rcu(mn, n, t,
> > +					  &mm->mmu_notifier.head, hlist) {
> > +			hlist_del_rcu(&mn->hlist);
> 
> This will race and kernel crash against mmu_notifier_register in
> SMP. You should resurrect the per-mmu_notifier_head lock in my last
> patch (except it can be converted from a rwlock_t to a regular
> spinlock_t) and drop the mmap_sem from
> mmu_notifier_register/unregister.

Agree.

That will also resolve the problem we discussed yesterday. 
I want to unregister my mmu_notifier when a GRU segment is
unmapped. This would not necessarily be at task termination.

However, the mmap_sem is already held for write by the core
VM at the point I would call the unregister function.
Currently, there is no __mmu_notifier_unregister() defined.

Moving to a different lock solves the problem.


-- jack

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