linux-kernel - Re: NFS lockdep lock misordering mmap_sem<->i_mutex

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Date:	Tue, 15 Dec 2009 22:21:34 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-kernel@...r.kernel.org, Trond.Myklebust@...app.com
Subject: Re: NFS lockdep lock misordering mmap_sem<->i_mutex_key with
	2.6.32-git1

On Mon, Dec 07, 2009 at 02:20:09PM +0100, Andi Kleen wrote:
> >    nfs_readdir
> >      nfs_do_filldir
> >        filldir
> >          copy_to_user
> >            [page_fault]			[grab mmap_sem]
> > 
> >  sys_mmap				[grab mmap_sem]
> >    do_mmap_pgoff
> >      mmap_region
> >        nfs_file_mmap
> >          nfs_revalidate_mapping  
> >            nfs_invalidate_mapping	[grab i_mutex]
> > 
> > I guess recent lockdep improvement find old bug.
> 
> Thanks for the analysis.
> 
> I guess should never do copy_*_user while holding i_mutex? There might
> be lots of cases like that.

No.  mmap_sem inside i_mutex is the normal order; NFS mmap is doing the
wrong thing here.  Note that readdir() vs. NFS (file-only, thankfully ;-)
mmap() is a non-issue; NFS mmap() vs. write() is much more interesting.

Again, a lot of mm/* code expects i_mutex, then mmap_sem order.  It's not
just readdir().
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