linux-kernel - Re: [patch 01/10] vfs: add path_create() and path

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Date:	Fri, 04 Apr 2008 13:04:53 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	ezk@...sunysb.edu
CC:	viro@...iv.linux.org.uk, ezk@...sunysb.edu,
	trond.myklebust@....uio.no, miklos@...redi.hu,
	akpm@...ux-foundation.org, dave@...ux.vnet.ibm.com,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 01/10] vfs: add path_create() and path_mknod()

> In message <20080403023239.GY9785@...IV.linux.org.uk>, Al Viro writes:
> > On Wed, Apr 02, 2008 at 10:21:24PM -0400, Erez Zadok wrote:
> > > Yes, I do grab both vfsmount and superblock refs.  I found out that grabbing
> > > vfsmount refs wasn't enough to prevent "umount -l" from detaching the f/s on
> > > which I'm stacked on.  So now at mount time (or branch management time), I
> > > grab those super-refs, as I have them after a successful path_lookup.  And,
> > > since I keep a list of the branches I'm stacked on, I know precisely which
> > > superblocks' references I need to release when unionfs is unmounted.
> > 
> > How the devil would holding a superblock prevent umount -l?
> 
> It doesn't prevent umount -l; but it prevents the lower superblock from
> being kfree()d if there were no references left to it, so I don't try to
> accessed freed memory (and get 6b6b6b6b oopses :-)

Having a ref on the vfsmount should be enough, regardless of whether
the mount is attached or not.  There's nothing special about a
detached mount, that anything outside namespace.c should care about.

Miklos
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