linux-kernel - Re: [PATCH 01 of 11] mmu-notifier-core

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Date:	Mon, 5 May 2008 11:21:13 -0500
From:	Jack Steiner <steiner@....com>
To:	Andrea Arcangeli <andrea@...ranet.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <clameter@....com>,
	Robin Holt <holt@....com>, Nick Piggin <npiggin@...e.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	kvm-devel@...ts.sourceforge.net,
	Kanoj Sarcar <kanojsarcar@...oo.com>,
	Roland Dreier <rdreier@...co.com>,
	Steve Wise <swise@...ngridcomputing.com>,
	linux-kernel@...r.kernel.org, Avi Kivity <avi@...ranet.com>,
	linux-mm@...ck.org, general@...ts.openfabrics.org,
	Hugh Dickins <hugh@...itas.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Anthony Liguori <aliguori@...ibm.com>,
	Chris Wright <chrisw@...hat.com>,
	Marcelo Tosatti <marcelo@...ck.org>,
	Eric Dumazet <dada1@...mosbay.com>,
	"Paul E. McKenney" <paulmck@...ibm.com>
Subject: Re: [PATCH 01 of 11] mmu-notifier-core

On Fri, May 02, 2008 at 05:05:04PM +0200, Andrea Arcangeli wrote:
> # HG changeset patch
> # User Andrea Arcangeli <andrea@...ranet.com>
> # Date 1209740175 -7200
> # Node ID 1489529e7b53d3f2dab8431372aa4850ec821caa
> # Parent  5026689a3bc323a26d33ad882c34c4c9c9a3ecd8
> mmu-notifier-core
 


I upgraded to the latest mmu notifier patch & hit a deadlock. (Sorry -
I should have seen this  earlier but I haven't tracked the last couple
of patches).

The GRU does the registration/deregistration of mmu notifiers from mmap/munmap.
At this point, the mmap_sem is already held writeable. I hit a deadlock
in mm_lock.

A quick fix would be to do one of the following:

	- move the mmap_sem locking to the caller of the [de]registration routines.
	  Since the first/last thing done in mm_lock/mm_unlock is to
	  acquire/release mmap_sem, this change does not cause major changes.

	- add a flag to mmu_notifier_[un]register routines to indicate
	  if mmap_sem is already locked.


I've temporarily deleted the mm_lock locking of mmap_sem and am continuing to
test. More later....


--- jack
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