linux-kernel - Re: [PATCH] dcache: fix d_splice

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Date:	Thu, 16 Jan 2014 11:10:16 -0500
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-nfs@...r.kernel.org, Miklos Szeredi <mszeredi@...e.cz>,
	Steven Whitehouse <swhiteho@...hat.com>
Subject: Re: [PATCH] dcache: fix d_splice_alias handling of aliases

On Wed, Jan 15, 2014 at 10:17:49AM -0500, bfields wrote:
> From: "J. Bruce Fields" <bfields@...hat.com>
> 
> d_splice_alias can create duplicate directory aliases (in the !new
> case), or (in the new case) d_move without holding appropriate locks.
> 
> d_materialise_unique deals with both of these problems.  (The latter
> seems to be dealt by trylocks (see __d_unalias), which look like they
> could cause spurious lookup failures--but that's at least better than
> corrupting the dcache.)
> 
> Signed-off-by: J. Bruce Fields <bfields@...hat.com>
> ---
>  fs/dcache.c |   25 +------------------------
>  1 file changed, 1 insertion(+), 24 deletions(-)
> 
> Only lightly tested....  If this is right, then we can also just ditch
> d_splice_alias completely, and clean up the various d_find_alias's.
> 
> I think the only reason we have both d_splice_alias and
> d_materialise_unique is that the former was written for exportable
> filesystems and the latter for distributed filesystems.
> 
> But we have at least one exportable filesystem (fuse) using
> d_materialise_unique.  And I doubt d_splice_alias was ever completely
> correct even for on-disk filesystems.
> 
> Am I missing some subtlety?

Hm, I just noticed:

    commit 0d0d110720d7960b77c03c9f2597faaff4b484ae
    Author: Miklos Szeredi <mszeredi@...e.cz>
    Date:   Mon Sep 16 14:52:00 2013 +0200

    GFS2: d_splice_alias() can't return error
    
    unless it was given an IS_ERR(inode), which isn't the case here.  So clean
    up the unnecessary error handling in gfs2_create_inode().
    
    This paves the way for real fixes (hence the stable Cc).
    
    Signed-off-by: Miklos Szeredi <mszeredi@...e.cz>
    Signed-off-by: Steven Whitehouse <swhiteho@...hat.com>
    Cc: stable@...r.kernel.org

While the statement is true for the current implementation of
d_splice_alias, I don't think it's actually true for any correct
implementation of d_splice_alias, which must be able to return at least
-ELOOP in the directory case.  Does gfs2 need fixing?

--b.
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