linux-kernel - Re: EMM: disable other notifiers before register and unregister

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Date:	Fri, 4 Apr 2008 14:30:40 +0200
From:	Andrea Arcangeli <andrea@...ranet.com>
To:	Christoph Lameter <clameter@....com>
Cc:	Hugh Dickins <hugh@...itas.com>, 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>,
	general@...ts.openfabrics.org,
	Steve Wise <swise@...ngridcomputing.com>,
	Roland Dreier <rdreier@...co.com>,
	Kanoj Sarcar <kanojsarcar@...oo.com>, steiner@....com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	daniel.blueman@...drics.com, Nick Piggin <npiggin@...e.de>
Subject: Re: EMM: disable other notifiers before register and unregister

On Thu, Apr 03, 2008 at 12:20:41PM -0700, Christoph Lameter wrote:
> On Thu, 3 Apr 2008, Andrea Arcangeli wrote:
> 
> > My attempt to fix this once and for all is to walk all vmas of the
> > "mm" inside mmu_notifier_register and take all anon_vma locks and
> > i_mmap_locks in virtual address order in a row. It's ok to take those
> > inside the mmap_sem. Supposedly if anybody will ever take a double
> > lock it'll do in order too. Then I can dump all the other locking and
> 
> What about concurrent mmu_notifier registrations from two mm_structs 
> that have shared mappings? Isnt there a potential deadlock situation?

No, the ordering of the lock avoids that. Here a snippnet.

/*
 * This operation locks against the VM for all pte/vma/mm related
 * operations that could ever happen on a certain mm. This includes
 * vmtruncate, try_to_unmap, and all page faults. The holder
 * must not hold any mm related lock. A single task can't take more
 * than one mm lock in a row or it would deadlock.
 */

So you can't do:

   mm_lock(mm1);
   mm_lock(mm2);

But if two different tasks run the mm_lock everything is ok. Each task
in the system can lock at most 1 mm at time.

> Well good luck. Hopefully we will get to something that works.

Looks good so far but I didn't finish it yet.
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