linux-kernel - Re: Q: NFSD readdir in linux-2.6.28

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Date:	Thu, 19 Mar 2009 15:17:17 +0000
From:	David Woodhouse <dwmw2@...radead.org>
To:	"hooanon05@...oo.co.jp" <hooanon05@...oo.co.jp>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: Q: NFSD readdir in linux-2.6.28

On Thu, 2009-03-19 at 14:54 +0000, hooanon05@...oo.co.jp wrote:
> 
> Hello David and Al,
> I have a question about NFSD readdir.
> 
> By the commit 14f7dd632011bb89c035722edd6ea0d90ca6b078
> "[PATCH] Copy XFS readdir hack into nfsd code", nfsd_buffered_filldir()
> was introduced and nfs3svc_encode_entry_plus() (the 'func' parameter) is
> not called from vfs_readdir().
> 
> In 2.6.27, when nfs3svc_encode_entry_plus() calls lookup_one_len(), the
> i_mutex lock was acquired by vfs_readdir() and it was not a problem.
> 
> After the commit (above), nfsd_readdir/nfsd_buffered_readdir/vfs_readdir
> calls nfsd_buffered_filldir(), and nfs3svc_encode_entry_plus() is called
> later.
> In this sequence, lookup_one_len() is called without i_mutex held.
> 
> Isn't it a problem?

Yes, well spotted. It didn't matter when the buffered readdir() was
purely internal to XFS, because it didn't matter there that we called
->lookup() without i_mutex set. But now we're exposing arbitrary file
systems to it, we need to make sure we follow the locking rules. 

I _think_ it's sufficient to make the affected callers of
lookup_one_len() lock the parent's i_mutex for themselves before calling
it. I'll take a closer look...

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@...el.com                              Intel Corporation

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