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

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Date:	Thu, 16 Jan 2014 16:15:42 +0000
From:	Steven Whitehouse <swhiteho@...hat.com>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	Al Viro <viro@...iv.linux.org.uk>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-nfs@...r.kernel.org,
	Miklos Szeredi <mszeredi@...e.cz>
Subject: Re: [PATCH] dcache: fix d_splice_alias handling of aliases

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@...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.

Yes, in that case, probably in two places,

Steve.


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