linux-kernel - Re: [PATCH 21/32] union-mount: Make lookup work for union-mounted file systems

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Message-Id: <E1M6iv8-0003CB-4O@pomaz-ex.szeredi.hu>
Date:	Wed, 20 May 2009 12:21:10 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	vaurora@...hat.com
CC:	miklos@...redi.hu, jblunck@...e.de, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, viro@...iv.linux.org.uk,
	bharata@...ibm.com, dwmw2@...radead.org, mszeredi@...e.cz
Subject: Re: [PATCH 21/32] union-mount: Make lookup work for union-mounted file systems

On Tue, 19 May 2009, Valerie Aurora wrote:
> On Tue, May 19, 2009 at 06:15:52PM +0200, Miklos Szeredi wrote:

> > Worse, it looks like there are still i_mutex lock ordering issues
> > (__hash_lookup_topmost()/__hash_lookup_build_union()).  What happens
> > if two separate unions of two filesystems are built where the order of
> > branches is reversed?
> 
> We have a similar problem in union_copyup_dir().  Hm, thinking about
> this, only one of the file systems can actually change while we are
> doing work.  That might help us get out of the lock ordering problems.
> Thoughts?

Right, we talked about this with Jan, and came basically to the same
conclusion.  The lookup on the lower branches needs to be separate
from the atomic lookup/create on the top branch.  Which means some
restructuring in the callers...

I'm not sure how all this could be simplified, I don't yet even
understand what all these different lookup functions are meant to do
(header comments might help).

Thanks,
Miklos
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