linux-kernel - Re: [PATCH 3/3] nfs: use ->mmap_prepare() to avoid an AB-BA deadlock

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Message-Id: <1195075905.22457.3.camel@lappy>
Date:	Wed, 14 Nov 2007 22:31:45 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Nick Piggin <npiggin@...e.de>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-arch@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Trond Myklebust <trond.myklebust@....uio.no>,
	Hugh Dickins <hugh@...itas.com>
Subject: Re: [PATCH 3/3] nfs: use ->mmap_prepare() to avoid an AB-BA
	deadlock


On Wed, 2007-11-14 at 22:22 +0100, Nick Piggin wrote:
> On Wed, Nov 14, 2007 at 09:01:39PM +0100, Peter Zijlstra wrote:
> > Normal locking order is:
> > 
> >   i_mutex
> >     mmap_sem
> > 
> > However NFS's ->mmap hook, which is called under mmap_sem, can take i_mutex.
> > Avoid this potential deadlock by doing the work that requires i_mutex from
> > the new ->mmap_prepare().
> > 
> > [ Is this sufficient, or does it introduce a race? ]
> 
> Seems like an OK patchset in my opinion. I don't know much about NFS
> unfortunately, but I wonder what prevents the condition fixed by
> nfs_revalidate_mapping from happening again while the mmap is active...?

As the changelog might have suggested, I'm not overly sure of the nfs
requirements myself. I think it just does a best effort at getting the
pages coherent with other clients, and then hopes for the best.

I'll let Trond enlighten us further before I make an utter fool of
myself :-)

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