linux-kernel - Re: [RFC][PATCH 2/3] vfs: Add a function to lazily unmount all mounts from any dentry.

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Message-ID: <20131008135648.GH14242@tucsk.piliscsaba.szeredi.hu>
Date:	Tue, 8 Oct 2013 15:56:48 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Rob Landley <rob@...dley.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC][PATCH 2/3] vfs: Add a function to lazily unmount all
 mounts from any dentry.

On Fri, Oct 04, 2013 at 03:43:18PM -0700, Eric W. Biederman wrote:
> 
> Signed-off-by: Eric W. Biederman <ebiederman@...tter.com>
> ---
>  fs/mount.h     |    1 +
>  fs/namespace.c |   24 ++++++++++++++++++++++++
>  2 files changed, 25 insertions(+), 0 deletions(-)
> 
> diff --git a/fs/mount.h b/fs/mount.h
> index e4342b8dfab1..7a6a2bb3f290 100644
> --- a/fs/mount.h
> +++ b/fs/mount.h
> @@ -79,6 +79,7 @@ static inline int is_mounted(struct vfsmount *mnt)
>  }
>  
>  extern struct mount *__lookup_mnt(struct vfsmount *, struct dentry *, int);
> +extern void detach_mounts(struct dentry *dentry);
>  
>  static inline void get_mnt_ns(struct mnt_namespace *ns)
>  {
> diff --git a/fs/namespace.c b/fs/namespace.c
> index d092964fe7f9..8eaee0c14fdb 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -1294,6 +1294,30 @@ static int do_umount(struct mount *mnt, int flags)
>  	return retval;
>  }
>  
> +void detach_mounts(struct dentry *dentry)
> +{
> +	struct mount *mnt, *next;
> +	struct mountpoint *mp;
> +
> +	namespace_lock();
> +	if (!d_mountpoint(dentry)) {
> +		namespace_unlock();
> +		return;
> +	}
> +	mp = new_mountpoint(dentry);
> +	if (IS_ERR(mp)) {
> +		return;
> +	}
> +	br_write_lock(&vfsmount_lock);
> +	list_for_each_entry_safe(mnt, next, &mp->m_list, mnt_mp_list) {

I don't think list_for_each_entry_safe is actually safe enough here.  Because
propageted umounts will likely remove other mounts from the same mountpoint as
well.  Doing it with "while (!list_empty(&mp->m_list))" should be better, and
AFAICS it will always make progress.


> +		if (!list_empty(&mnt->mnt_list))
> +			umount_tree(mnt, 1);
> +	}
> +	br_write_unlock(&vfsmount_lock);
> +	put_mountpoint(mp);
> +	namespace_unlock();
> +}
> +
>  /* 
>   * Is the caller allowed to modify his namespace?
>   */
> -- 
> 1.7.5.4
> 
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