linux-kernel - Re: [GIT PULL] 9p file system bug fixes for 2.6.35-rc2

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Message-ID: <20100608004102.GQ31073@ZenIV.linux.org.uk>
Date:	Tue, 8 Jun 2010 01:41:02 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Eric Van Hensbergen <ericvh@...il.com>,
	V9FS Developers <v9fs-developer@...ts.sourceforge.net>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PULL] 9p file system bug fixes for 2.6.35-rc2

On Mon, Jun 07, 2010 at 05:08:19PM -0700, Linus Torvalds wrote:

> In fact, the other thing that I find doing that whole "dentry->d_parent" 
> thing seems to literally be broken. If you look at v9fs_fid_lookup(), 
> you'll notice how it walks up the d_parent chain, and at that point you do 
> NOT own the directory i_mutex, so at that point d_parent really _can_ be 
> changing wildly due to concurrent renames or whatever.

Eh...  It's bogus, all right, but i_mutex is not the correct solution.
You'd have to take it on a lot of inodes along the way to root *and*
you'd violate the ordering in process (ancestors first).

I'm not sure what's the right thing to do there, actually - s_vfs_rename_sem
also won't do, since it'll give you ordering problems of its own (it's
taken before i_mutex in VFS, so trying to take it under i_mutex would not
do).

The _really_ interesting question is how do servers deal with topology-changing
renames.  Note that the problem exists only with extended 9P - with the
original one all of that had been a non-issue, since it didn't allow
cross-directory renames at all and the tree topology remained stable all along.
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