Re: [PATCH]: Factor out PTY index allocation

["Serge E. Hallyn" <serue@us.ibm.com>]

Quoting sukadev@us.ibm.com (sukadev@us.ibm.com):
> We noticed this while working on pts namespaces and believe this might
> be an useful change even as we rework our pts/device namespace approach.
> 
> ---
> 
> From: Sukadev Bhattiprolu <sukadev@us.ibm.com>
> Subject: [PATCH]: Factor out PTY index allocation
> 
> Factor out the code used to allocate/free a pts index into new interfaces,
> devpts_new_index() and devpts_kill_index().  This localizes the external
> data structures used in managing the pts indices.
> 
> Signed-off-by: Sukadev Bhattiprolu <sukadev@us.ibm.com>
> Signed-off-by: Serge Hallyn<serue@us.ibm.com>
> Signed-off-by: Matt Helsley<matthltc@us.ibm.com>

No traces of devpts namespaces here, so I assume this should be
non-offensive and fine for inclusion.

thanks,
-serge

> ---
>  drivers/char/tty_io.c     |   40 ++++++----------------------------------
>  fs/devpts/inode.c         |   42 +++++++++++++++++++++++++++++++++++++++++-
>  include/linux/devpts_fs.h |    4 ++++
>  3 files changed, 51 insertions(+), 35 deletions(-)
> 
> Index: 2.6.25-rc8-mm2/include/linux/devpts_fs.h
> ===================================================================
> --- 2.6.25-rc8-mm2.orig/include/linux/devpts_fs.h	2008-01-26 09:49:16.000000000 -0800
> +++ 2.6.25-rc8-mm2/include/linux/devpts_fs.h	2008-04-16 09:51:15.000000000 -0700
> @@ -17,6 +17,8 @@
> 
>  #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 */
> @@ -24,6 +26,8 @@ void devpts_pty_kill(int number);		 /* u
>  #else
> 
>  /* Dummy stubs in the no-pty case */
> +static inline int devpts_new_index(void) { return -EINVAL; }
> +static inline void devpts_kill_index(int idx) { }
>  static inline int devpts_pty_new(struct tty_struct *tty) { return -EINVAL; }
>  static inline struct tty_struct *devpts_get_tty(int number) { return NULL; }
>  static inline void devpts_pty_kill(int number) { }
> Index: 2.6.25-rc8-mm2/drivers/char/tty_io.c
> ===================================================================
> --- 2.6.25-rc8-mm2.orig/drivers/char/tty_io.c	2008-04-16 09:51:11.000000000 -0700
> +++ 2.6.25-rc8-mm2/drivers/char/tty_io.c	2008-04-16 09:51:15.000000000 -0700
> @@ -91,7 +91,6 @@
>  #include <linux/module.h>
>  #include <linux/smp_lock.h>
>  #include <linux/device.h>
> -#include <linux/idr.h>
>  #include <linux/wait.h>
>  #include <linux/bitops.h>
>  #include <linux/delay.h>
> @@ -137,9 +136,6 @@ EXPORT_SYMBOL(tty_mutex);
> 
>  #ifdef CONFIG_UNIX98_PTYS
>  extern struct tty_driver *ptm_driver;	/* Unix98 pty masters; for /dev/ptmx */
> -extern int pty_limit;			/* Config limit on Unix98 ptys */
> -static DEFINE_IDR(allocated_ptys);
> -static DEFINE_MUTEX(allocated_ptys_lock);
>  static int ptmx_open(struct inode *, struct file *);
>  #endif
> 
> @@ -2636,15 +2632,9 @@ static void release_dev(struct file *fil
>  	 */
>  	release_tty(tty, idx);
> 
> -#ifdef CONFIG_UNIX98_PTYS
>  	/* Make this pty number available for reallocation */
> -	if (devpts) {
> -		mutex_lock(&allocated_ptys_lock);
> -		idr_remove(&allocated_ptys, idx);
> -		mutex_unlock(&allocated_ptys_lock);
> -	}
> -#endif
> -
> +	if (devpts)
> +		devpts_kill_index(idx);
>  }
> 
>  /**
> @@ -2800,29 +2790,13 @@ static int ptmx_open(struct inode *inode
>  	struct tty_struct *tty;
>  	int retval;
>  	int index;
> -	int idr_ret;
> 
>  	nonseekable_open(inode, filp);
> 
>  	/* find a device that is not in use. */
> -	mutex_lock(&allocated_ptys_lock);
> -	if (!idr_pre_get(&allocated_ptys, GFP_KERNEL)) {
> -		mutex_unlock(&allocated_ptys_lock);
> -		return -ENOMEM;
> -	}
> -	idr_ret = idr_get_new(&allocated_ptys, NULL, &index);
> -	if (idr_ret < 0) {
> -		mutex_unlock(&allocated_ptys_lock);
> -		if (idr_ret == -EAGAIN)
> -			return -ENOMEM;
> -		return -EIO;
> -	}
> -	if (index >= pty_limit) {
> -		idr_remove(&allocated_ptys, index);
> -		mutex_unlock(&allocated_ptys_lock);
> -		return -EIO;
> -	}
> -	mutex_unlock(&allocated_ptys_lock);
> +	index = devpts_new_index();
> +	if (index < 0)
> +		return index;
> 
>  	mutex_lock(&tty_mutex);
>  	retval = init_dev(ptm_driver, index, &tty);
> @@ -2847,9 +2821,7 @@ out1:
>  	release_dev(filp);
>  	return retval;
>  out:
> -	mutex_lock(&allocated_ptys_lock);
> -	idr_remove(&allocated_ptys, index);
> -	mutex_unlock(&allocated_ptys_lock);
> +	devpts_kill_index(index);
>  	return retval;
>  }
>  #endif
> Index: 2.6.25-rc8-mm2/fs/devpts/inode.c
> ===================================================================
> --- 2.6.25-rc8-mm2.orig/fs/devpts/inode.c	2008-02-27 15:17:59.000000000 -0800
> +++ 2.6.25-rc8-mm2/fs/devpts/inode.c	2008-04-16 09:51:15.000000000 -0700
> @@ -17,6 +17,7 @@
>  #include <linux/namei.h>
>  #include <linux/mount.h>
>  #include <linux/tty.h>
> +#include <linux/idr.h>
>  #include <linux/devpts_fs.h>
>  #include <linux/parser.h>
>  #include <linux/fsnotify.h>
> @@ -26,6 +27,10 @@
> 
>  #define DEVPTS_DEFAULT_MODE 0600
> 
> +extern int pty_limit;			/* Config limit on Unix98 ptys */
> +static DEFINE_IDR(allocated_ptys);
> +static DECLARE_MUTEX(allocated_ptys_lock);
> +
>  static struct vfsmount *devpts_mnt;
>  static struct dentry *devpts_root;
> 
> @@ -171,9 +176,44 @@ static struct dentry *get_node(int num)
>  	return lookup_one_len(s, root, sprintf(s, "%d", num));
>  }
> 
> +int devpts_new_index(void)
> +{
> +	int index;
> +	int idr_ret;
> +
> +retry:
> +	if (!idr_pre_get(&allocated_ptys, GFP_KERNEL)) {
> +		return -ENOMEM;
> +	}
> +
> +	down(&allocated_ptys_lock);
> +	idr_ret = idr_get_new(&allocated_ptys, NULL, &index);
> +	if (idr_ret < 0) {
> +		up(&allocated_ptys_lock);
> +		if (idr_ret == -EAGAIN)
> +			goto retry;
> +		return -EIO;
> +	}
> +
> +	if (index >= pty_limit) {
> +		idr_remove(&allocated_ptys, index);
> +		up(&allocated_ptys_lock);
> +		return -EIO;
> +	}
> +	up(&allocated_ptys_lock);
> +	return index;
> +}
> +
> +void devpts_kill_index(int idx)
> +{
> +	down(&allocated_ptys_lock);
> +	idr_remove(&allocated_ptys, idx);
> +	up(&allocated_ptys_lock);
> +}
> +
>  int devpts_pty_new(struct tty_struct *tty)
>  {
> -	int number = tty->index;
> +	int number = tty->index; /* tty layer puts index from devpts_new_index() in here */
>  	struct tty_driver *driver = tty->driver;
>  	dev_t device = MKDEV(driver->major, driver->minor_start+number);
>  	struct dentry *dentry;