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

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Message-ID: <20110322201101.GZ22723@ZenIV.linux.org.uk>
Date:	Tue, 22 Mar 2011 20:11:01 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	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, 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
   proceeded to copy lower into new location
   set dentry to very odd (upper,lower) pair
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