Re: [PATCH] dcache: fix d_splice_alias handling of aliases

From: J. Bruce Fields
Date: Thu Jan 16 2014 - 11:10:27 EST

On Wed, Jan 15, 2014 at 10:17:49AM -0500, bfields wrote:
> From: "J. Bruce Fields" <bfields@xxxxxxxxxx>
> 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@xxxxxxxxxx>
> ---
> 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@xxxxxxx>
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@xxxxxxx>
Signed-off-by: Steven Whitehouse <swhiteho@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx

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?

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