Date: Fri, 17 Feb 2006 17:54:28 -0800
From: Andrew Morton <akpm@osdl.org>
To: Herbert Poetzl <herbert@13thfloor.at>
Cc: viro@ftp.linux.org.uk, linux-kernel@vger.kernel.org
Subject: Re: kjournald keeps reference to namespace
Message-Id: <20060217175428.7ce7b26f.akpm@osdl.org>
In-Reply-To: <20060218013547.GA32706@MAIL.13thfloor.at>
References: <20060218013547.GA32706@MAIL.13thfloor.at>
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Herbert Poetzl <herbert@13thfloor.at> wrote:
>
> 
> Hi Folks!
> 
> when creating a private namespace (CLONE_NS) and
> then mounting an ext3 filesystem, a new kernel
> thread (kjournald) is created, which keeps a
> reference to the namespace, which after the the
> process exits, remains and blocks access to the
> block device, as it is still bd_claim-ed.

There are numerous ways in which user processes can parent kernel threads.

bix:/usr/src/linux-2.6.16-rc4> grep -rl kernel_thread drivers net fs | wc
     64      64    1657

> this leaves a private namespace behind and a
> block device which cannot be opened exclusively.
> unmount is not an option, as the namespace is
> not longer reachable.
> 
> this behaviour seems to be there since ever,
> well since namespaces and kjournald exists :)
> 
> the following 'cruel' hack 'solves' this issue
> 
> best,
> Herbert
> 
> 
> --- fs/jbd/journal.c.orig	2006-01-03 17:29:56 +0100
> +++ fs/jbd/journal.c	2006-02-18 02:23:21 +0100
> @@ -33,6 +33,7 @@
>  #include <linux/mm.h>
>  #include <linux/suspend.h>
>  #include <linux/pagemap.h>
> +#include <linux/namespace.h>
>  #include <asm/uaccess.h>
>  #include <asm/page.h>
>  #include <linux/proc_fs.h>
> @@ -116,6 +117,13 @@ static int kjournald(void *arg)
>  	struct timer_list timer;
>  
>  	daemonize("kjournald");
> +	{
> +		struct namespace *ns = current->namespace;
> +
> +		current->namespace = NULL;
> +		put_namespace(ns);
> +	}
> +
>  

I think it'd be better to convert ext3 to use the kthread API which appears
to accidentally not have this problem, because such threads are parented by
keventd, which were parented by init.

That being said, perhaps we should do a put_namespace() in kernel_thread(),
too.

I'm kinda surprised that your patch didn't oops over a NULL ->namespace
when the kernel internally mounted the root filesystem.

-
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