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

From: Steven Whitehouse
Date: Thu Jan 16 2014 - 11:16:40 EST


Hi,

On Thu, 2014-01-16 at 11:10 -0500, J. Bruce Fields wrote:
> 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?
>
> --b.

Yes, in that case, probably in two places,

Steve.


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