linux-kernel - Re: [git pull] vfs and fs fixes

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Date:	Tue, 17 Apr 2012 12:22:16 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Al Viro <viro@...iv.linux.org.uk>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [git pull] vfs and fs fixes

On Tue, Apr 17, 2012 at 08:01:48AM -0700, Linus Torvalds wrote:
> On Mon, Apr 16, 2012 at 10:25 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> >        A bunch of endianness fixes plus a patch from bfields untangling
> > dependencies between vfs and nfsd trees; in principle, we could keep it
> > in nfsd tree (along with a bunch of followups that definitely belong there),
> > but Miklos' stuff in fs/namei.c steps fairly close to it and overlayfs
> > and unionfs series - even closer, so that would create serious PITA for
> > both, whichever tree it would sit in.
> 
> Why is that double mutex taking in vfs_rename_other() safe from ABBA?
> 
> We aren't guaranteed to hold the s_vfs_rename_mutex, since the parent
> directories may be the same.
> 
> And yes, we hold the i_mutex on that shared parent, but the inodes may
> exist (hardlinked) in another directory, so another rename could be
> doing the i_mutex in the reverse order.
> 
> Maybe there is some reason why that double lock is safe, but I don't
> see it, and I want it clearly documented. So I'm not pulling this.

Ugh, no, I think you're right:

	rename A/a->A/b
	rename B/b->B/b

where A/a and B/a are the same file, and A/b and B/b are the same file,
can result in the first rename holding the lock on A and a and waiting
on b, and the second holding the lock on B and b and waiting on a.

--b.
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