Quoting sukadev@us.ibm.com (sukadev@us.ibm.com):
> 
> From: Sukadev Bhattiprolu <sukadev@us.ibm.com>
> Subject: [PATCH 4/7]: Implement get_pts_ns() and put_pts_ns()
> 
> Implement get_pts_ns() and put_pts_ns() interfaces.
> 
> Signed-off-by: Sukadev Bhattiprolu <sukadev@us.ibm.com>
> ---
>  include/linux/devpts_fs.h |   21 ++++++++++++++++++++-
>  1 file changed, 20 insertions(+), 1 deletion(-)
> 
> Index: 2.6.25-rc5-mm1/include/linux/devpts_fs.h
> ===================================================================
> --- 2.6.25-rc5-mm1.orig/include/linux/devpts_fs.h	2008-03-24 20:04:31.000000000 -0700
> +++ 2.6.25-rc5-mm1/include/linux/devpts_fs.h	2008-03-24 20:05:05.000000000 -0700
> @@ -27,13 +27,26 @@ struct pts_namespace {
>  extern struct pts_namespace init_pts_ns;
> 
>  #ifdef CONFIG_UNIX98_PTYS
> -
>  int devpts_new_index(void);
>  void devpts_kill_index(int idx);
>  int devpts_pty_new(struct tty_struct *tty);      /* mknod in devpts */
>  struct tty_struct *devpts_get_tty(int number);	 /* get tty structure */
>  void devpts_pty_kill(int number);		 /* unlink */
> 
> +static inline void free_pts_ns(struct kref *ns_kref) { }
> +
> +static inline struct pts_namespace *get_pts_ns(struct pts_namespace *ns)
> +{
> +	if (ns && (ns != &init_pts_ns))
> +		kref_get(&ns->kref);
> +	return ns;
> +}
> +static inline void put_pts_ns(struct pts_namespace *ns)
> +{
> +	if (ns && (ns != &init_pts_ns))
> +		kref_put(&ns->kref, free_pts_ns);

This isn't right, or I'm not thinking right.  Don't you somewhere
need to

	1. rcu_assign ns->mnt->mnt_sb->s_fs_info to NULL
	2. wait a grace period
	3. call free_pts_ns and check the refcount on the ns again?

and then do pts_ns_from_inode() under an rcu_read_lock and grab
a ref to the ns?

> +}
> +
>  #else
> 
>  /* Dummy stubs in the no-pty case */
> @@ -43,6 +56,12 @@ static inline int devpts_pty_new(struct 
>  static inline struct tty_struct *devpts_get_tty(int number) { return NULL; }
>  static inline void devpts_pty_kill(int number) { }
> 
> +static inline struct pts_namespace *get_pts_ns(struct pts_namespace *ns)
> +{
> +	return &init_pts_ns;
> +}
> +
> +static inline void put_pts_ns(struct pts_namespace *ns) { }
>  #endif
> 
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