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Date: Thu, 1 Mar 2012 09:10:10 -0500
From: Christoph Hellwig <hch@infradead.org>
To: Gleb Kurtsou <gleb.kurtsou@gmail.com>
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References: <20120203193719.GB3283@deviant.kiev.zoral.com.ua>
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Cc: Attilio Rao <attilio@freebsd.org>,
	Konstantin Belousov <kostikbel@gmail.com>,
	Pawel Jakub Dawidek <pjd@FreeBSD.org>, arch@freebsd.org
Subject: Re: Prefaulting for i/o buffers
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On Thu, Mar 01, 2012 at 01:16:24PM +0200, Gleb Kurtsou wrote:
> Are you aware of a real "libraries for file systems" VFS example? It
> sounds very interesting but I'm afraid it's going to look good only in
> theory. E.g. locking at file system level (Darwin, Dragonfly BSD) looks
> rather messy (IMHO) and more likely to be bug prone. On the other side
> Linux has optional per file system rename lock making VOP_RENAME
> implementation much easier, while ours is tremendously difficult to do
> right.

All namespace locking in Linux is in the VFS, and it mandatory.  A
filesystem wide lock is only used for cross-directory renames.

A more detailed description is here:

	http://git.kernel.dk/?p=linux.git;a=blob;f=Documentation/filesystems/directory-locking