linux-kernel - Re: [PATCH 0/6 v7] overlay filesystem

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Date:	Tue, 22 Mar 2011 20:43:17 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	miklos@...redi.hu, torvalds@...ux-foundation.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, apw@...onical.com, nbd@...nwrt.org,
	neilb@...e.de
Subject: Re: [PATCH 0/6 v7] overlay filesystem - request for inclusion

On Tue, 22 Mar 2011, Al Viro wrote:
> On Tue, Mar 22, 2011 at 07:58:17PM +0100, Miklos Szeredi wrote:
> > On Tue, 22 Mar 2011, Linus Torvalds wrote:
> > > On Tue, Mar 22, 2011 at 11:39 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
> > > >
> > > > Locking analysis would be really nice; AFAICS, it violates locking order
> > > > when called from e.g. ->setattr()
> > 
> > Locking order is always:
> > 
> > -> overlayfs locks
> >    -> upper fs locks
> >    -> lower fs locks
> > 
> > So it's really pretty simple and easy to validate.
> 
> In which *order* on the upper fs?

In copy up it does:

-> lock parent on upper
  -> lock child on upper

So a setattr with copy up would go like this:

-> lock child on overlayfs
  -> lock parent on upper
     ->lock child on upper
  -> lock child on upper

> > Protection is exactly as for userspace callers.  AFAICT.
> 
> Pardon?  You traverse the chain of ancestors; fine, but who says it stays
> anywhere near being relevant as you go?

Not quite sure I understand.

There are no assumptions about locks in overlayfs keeping anything
relevant in upper/lower fs.  Everything is re-checked and re-locked on
the upper layer before proceeding with the rename.

Thanks,
Miklos

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