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

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Message-Id: <E1Q28N4-0001tv-O6@pomaz-ex.szeredi.hu>
Date:	Tue, 22 Mar 2011 21:40:06 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Miklos Szeredi <miklos@...redi.hu>
CC:	viro@...IV.linux.org.uk, 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, Miklos Szeredi wrote:
> On Tue, 22 Mar 2011, Al Viro wrote:
> > On Tue, Mar 22, 2011 at 09:06:38PM +0100, Miklos Szeredi wrote:
> > > On Tue, 22 Mar 2011, Al Viro wrote:
> > > > Proceeding with rename is not interesting; proceeding with copyup is.
> > > > 
> > > > Who said that by the time we get to copy_up_locked you will still have
> > > > dentry (and upper) match lowerpath?  Or that ->d_parent on overlay and
> > > > on upper will change in sync, for that matter - there are two d_move()
> > > > calls involved...
> > > 
> > > If rename is involved, than rename itself already did the copy up.
> > > And that's checked before proceeding with the actual copy up.  If
> > > there was no rename, then that guarantees that things are in sync, at
> > > least for the duration of the copy up.
> > 
> > What do you mean, before?  It's not atomic...  What happens if e.g.
> > you get
> > 
> > A: decided to do copy_up_locked
> >    blocked on i_mutex
> > 
> > B: did copy_up
> >    did rename(), complete with d_move()
> >    did unlink() in new place
> > 
> > A: got CPU back, got i_mutex
> 
> Here it can check if the file was copied up or not.  OK, I see the
> code doesn't quite get that right.
> 
> Patch below would fix it, I think.

Scratch that, d_move() on overlayfs would also have to be under
i_mutex of upper.  But that should be doable as well.  I'll fix that
tomorrow when my brain is a little fresher.

Thanks,
Miklos
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