text: #include <linux/types.h>
  #include <linux/errno.h>
  #include <linux/kmod.h>
  #include <linux/sched.h>
  #include <linux/interrupt.h>
  #include <linux/tty.h>
  #include <linux/tty_driver.h>
  #include <linux/file.h>
  #include <linux/mm.h>
  #include <linux/string.h>
  #include <linux/slab.h>
  #include <linux/poll.h>
  #include <linux/proc_fs.h>
  #include <linux/init.h>
  #include <linux/module.h>
  #include <linux/device.h>
  #include <linux/wait.h>
  #include <linux/bitops.h>
  #include <linux/seq_file.h>
  #include <linux/uaccess.h>
  #include <linux/ratelimit.h>
  
  #undef LDISC_DEBUG_HANGUP
  
  #ifdef LDISC_DEBUG_HANGUP
  #define tty_ldisc_debug(tty, f, args...) ({				       \
  	char __b[64];							       \
  	printk(KERN_DEBUG "%s: %s: " f, __func__, tty_name(tty, __b), ##args); \
  })
  #else
  #define tty_ldisc_debug(tty, f, args...)
  #endif
  
  /* lockdep nested classes for tty->ldisc_sem */
  enum {
  	LDISC_SEM_NORMAL,
  	LDISC_SEM_OTHER,
  };
  
  
  /*
   *	This guards the refcounted line discipline lists. The lock
   *	must be taken with irqs off because there are hangup path
   *	callers who will do ldisc lookups and cannot sleep.
   */
  
  static DEFINE_RAW_SPINLOCK(tty_ldiscs_lock);
  /* Line disc dispatch table */
  static struct tty_ldisc_ops *tty_ldiscs[NR_LDISCS];
  
  /**
   *	tty_register_ldisc	-	install a line discipline
   *	@disc: ldisc number
   *	@new_ldisc: pointer to the ldisc object
   *
   *	Installs a new line discipline into the kernel. The discipline
   *	is set up as unreferenced and then made available to the kernel
   *	from this point onwards.
   *
   *	Locking:
   *		takes tty_ldiscs_lock to guard against ldisc races
   */
  
  int tty_register_ldisc(int disc, struct tty_ldisc_ops *new_ldisc)
  {
  	unsigned long flags;
  	int ret = 0;
  
  	if (disc < N_TTY || disc >= NR_LDISCS)
  		return -EINVAL;
  
  	raw_spin_lock_irqsave(&tty_ldiscs_lock, flags);
  	tty_ldiscs[disc] = new_ldisc;
  	new_ldisc->num = disc;
  	new_ldisc->refcount = 0;
  	raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);
  
  	return ret;
  }
  EXPORT_SYMBOL(tty_register_ldisc);
  
  /**
   *	tty_unregister_ldisc	-	unload a line discipline
   *	@disc: ldisc number
   *	@new_ldisc: pointer to the ldisc object
   *
   *	Remove a line discipline from the kernel providing it is not
   *	currently in use.
   *
   *	Locking:
   *		takes tty_ldiscs_lock to guard against ldisc races
   */
  
  int tty_unregister_ldisc(int disc)
  {
  	unsigned long flags;
  	int ret = 0;
  
  	if (disc < N_TTY || disc >= NR_LDISCS)
  		return -EINVAL;
  
  	raw_spin_lock_irqsave(&tty_ldiscs_lock, flags);
  	if (tty_ldiscs[disc]->refcount)
  		ret = -EBUSY;
  	else
  		tty_ldiscs[disc] = NULL;
  	raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);
  
  	return ret;
  }
  EXPORT_SYMBOL(tty_unregister_ldisc);
  
  static struct tty_ldisc_ops *get_ldops(int disc)
  {
  	unsigned long flags;
  	struct tty_ldisc_ops *ldops, *ret;
  
  	raw_spin_lock_irqsave(&tty_ldiscs_lock, flags);
  	ret = ERR_PTR(-EINVAL);
  	ldops = tty_ldiscs[disc];
  	if (ldops) {
  		ret = ERR_PTR(-EAGAIN);
  		if (try_module_get(ldops->owner)) {
  			ldops->refcount++;
  			ret = ldops;
  		}
  	}
  	raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);
  	return ret;
  }
  
  static void put_ldops(struct tty_ldisc_ops *ldops)
  {
  	unsigned long flags;
  
  	raw_spin_lock_irqsave(&tty_ldiscs_lock, flags);
  	ldops->refcount--;
  	module_put(ldops->owner);
  	raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);
  }
  
  /**
   *	tty_ldisc_get		-	take a reference to an ldisc
   *	@disc: ldisc number
   *
   *	Takes a reference to a line discipline. Deals with refcounts and
   *	module locking counts. Returns NULL if the discipline is not available.
   *	Returns a pointer to the discipline and bumps the ref count if it is
   *	available
   *
   *	Locking:
   *		takes tty_ldiscs_lock to guard against ldisc races
   */
  
  static struct tty_ldisc *tty_ldisc_get(struct tty_struct *tty, int disc)
  {
  	struct tty_ldisc *ld;
  	struct tty_ldisc_ops *ldops;
  
  	if (disc < N_TTY || disc >= NR_LDISCS)
  		return ERR_PTR(-EINVAL);
  
  	/*
  	 * Get the ldisc ops - we may need to request them to be loaded
  	 * dynamically and try again.
  	 */
  	ldops = get_ldops(disc);
  	if (IS_ERR(ldops)) {
  		request_module("tty-ldisc-%d", disc);
  		ldops = get_ldops(disc);
  		if (IS_ERR(ldops))
  			return ERR_CAST(ldops);
  	}
  
  	ld = kmalloc(sizeof(struct tty_ldisc), GFP_KERNEL);
  	if (ld == NULL) {
  		put_ldops(ldops);
  		return ERR_PTR(-ENOMEM);
  	}
  
  	ld->ops = ldops;
  	ld->tty = tty;
  
  	return ld;
  }
  
  /**
   *	tty_ldisc_put		-	release the ldisc
   *
   *	Complement of tty_ldisc_get().
   */
  static inline void tty_ldisc_put(struct tty_ldisc *ld)
  {
  	if (WARN_ON_ONCE(!ld))
  		return;
  
  	put_ldops(ld->ops);
  	kfree(ld);
  }
  
  static void *tty_ldiscs_seq_start(struct seq_file *m, loff_t *pos)
  {
  	return (*pos < NR_LDISCS) ? pos : NULL;
  }
  
  static void *tty_ldiscs_seq_next(struct seq_file *m, void *v, loff_t *pos)
  {
  	(*pos)++;
  	return (*pos < NR_LDISCS) ? pos : NULL;
  }
  
  static void tty_ldiscs_seq_stop(struct seq_file *m, void *v)
  {
  }
  
  static int tty_ldiscs_seq_show(struct seq_file *m, void *v)
  {
  	int i = *(loff_t *)v;
  	struct tty_ldisc_ops *ldops;
  
  	ldops = get_ldops(i);
  	if (IS_ERR(ldops))
  		return 0;
  	seq_printf(m, "%-10s %2d\n", ldops->name ? ldops->name : "???", i);
  	put_ldops(ldops);
  	return 0;
  }
  
  static const struct seq_operations tty_ldiscs_seq_ops = {
  	.start	= tty_ldiscs_seq_start,
  	.next	= tty_ldiscs_seq_next,
  	.stop	= tty_ldiscs_seq_stop,
  	.show	= tty_ldiscs_seq_show,
  };
  
  static int proc_tty_ldiscs_open(struct inode *inode, struct file *file)
  {
  	return seq_open(file, &tty_ldiscs_seq_ops);
  }
  
  const struct file_operations tty_ldiscs_proc_fops = {
  	.owner		= THIS_MODULE,
  	.open		= proc_tty_ldiscs_open,
  	.read		= seq_read,
  	.llseek		= seq_lseek,
  	.release	= seq_release,
  };
  
  /**
   *	tty_ldisc_ref_wait	-	wait for the tty ldisc
   *	@tty: tty device
   *
   *	Dereference the line discipline for the terminal and take a
   *	reference to it. If the line discipline is in flux then
   *	wait patiently until it changes.
   *
   *	Note: Must not be called from an IRQ/timer context. The caller
   *	must also be careful not to hold other locks that will deadlock
   *	against a discipline change, such as an existing ldisc reference
   *	(which we check for)
   *
   *	Note: only callable from a file_operations routine (which
   *	guarantees tty->ldisc != NULL when the lock is acquired).
   */
  
  struct tty_ldisc *tty_ldisc_ref_wait(struct tty_struct *tty)
  {
  	ldsem_down_read(&tty->ldisc_sem, MAX_SCHEDULE_TIMEOUT);
  	WARN_ON(!tty->ldisc);
  	return tty->ldisc;
  }
  EXPORT_SYMBOL_GPL(tty_ldisc_ref_wait);
  
  /**
   *	tty_ldisc_ref		-	get the tty ldisc
   *	@tty: tty device
   *
   *	Dereference the line discipline for the terminal and take a
   *	reference to it. If the line discipline is in flux then
   *	return NULL. Can be called from IRQ and timer functions.
   */
  
  struct tty_ldisc *tty_ldisc_ref(struct tty_struct *tty)
  {
  	struct tty_ldisc *ld = NULL;
  
  	if (ldsem_down_read_trylock(&tty->ldisc_sem)) {
  		ld = tty->ldisc;
  		if (!ld)
  			ldsem_up_read(&tty->ldisc_sem);
  	}
  	return ld;
  }
  EXPORT_SYMBOL_GPL(tty_ldisc_ref);
  
  /**
   *	tty_ldisc_deref		-	free a tty ldisc reference
   *	@ld: reference to free up
   *
   *	Undoes the effect of tty_ldisc_ref or tty_ldisc_ref_wait. May
   *	be called in IRQ context.
   */
  
  void tty_ldisc_deref(struct tty_ldisc *ld)
  {
  	ldsem_up_read(&ld->tty->ldisc_sem);
  }
  EXPORT_SYMBOL_GPL(tty_ldisc_deref);
  
  
  static inline int __lockfunc
  tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout)
  {
  	return ldsem_down_write(&tty->ldisc_sem, timeout);
  }
  
  static inline int __lockfunc
  tty_ldisc_lock_nested(struct tty_struct *tty, unsigned long timeout)
  {
  	return ldsem_down_write_nested(&tty->ldisc_sem,
  				       LDISC_SEM_OTHER, timeout);
  }
  
  static inline void tty_ldisc_unlock(struct tty_struct *tty)
  {
  	return ldsem_up_write(&tty->ldisc_sem);
  }
  
  static int __lockfunc
  tty_ldisc_lock_pair_timeout(struct tty_struct *tty, struct tty_struct *tty2,
  			    unsigned long timeout)
  {
  	int ret;
  
  	if (tty < tty2) {
  		ret = tty_ldisc_lock(tty, timeout);
  		if (ret) {
  			ret = tty_ldisc_lock_nested(tty2, timeout);
  			if (!ret)
  				tty_ldisc_unlock(tty);
  		}
  	} else {
  		/* if this is possible, it has lots of implications */
  		WARN_ON_ONCE(tty == tty2);
  		if (tty2 && tty != tty2) {
  			ret = tty_ldisc_lock(tty2, timeout);
  			if (ret) {
  				ret = tty_ldisc_lock_nested(tty, timeout);
  				if (!ret)
  					tty_ldisc_unlock(tty2);
  			}
  		} else
  			ret = tty_ldisc_lock(tty, timeout);
  	}
  
  	if (!ret)
  		return -EBUSY;
  
  	set_bit(TTY_LDISC_HALTED, &tty->flags);
  	if (tty2)
  		set_bit(TTY_LDISC_HALTED, &tty2->flags);
  	return 0;
  }
  
  static void __lockfunc
  tty_ldisc_lock_pair(struct tty_struct *tty, struct tty_struct *tty2)
  {
  	tty_ldisc_lock_pair_timeout(tty, tty2, MAX_SCHEDULE_TIMEOUT);
  }
  
  static void __lockfunc tty_ldisc_unlock_pair(struct tty_struct *tty,
  					     struct tty_struct *tty2)
  {
  	tty_ldisc_unlock(tty);
  	if (tty2)
  		tty_ldisc_unlock(tty2);
  }
  
  static void __lockfunc tty_ldisc_enable_pair(struct tty_struct *tty,
  					     struct tty_struct *tty2)
  {
  	clear_bit(TTY_LDISC_HALTED, &tty->flags);
  	if (tty2)
  		clear_bit(TTY_LDISC_HALTED, &tty2->flags);
  
  	tty_ldisc_unlock_pair(tty, tty2);
  }
  
  /**
   *	tty_ldisc_flush	-	flush line discipline queue
   *	@tty: tty
   *
   *	Flush the line discipline queue (if any) for this tty. If there
   *	is no line discipline active this is a no-op.
   */
  
  void tty_ldisc_flush(struct tty_struct *tty)
  {
  	struct tty_ldisc *ld = tty_ldisc_ref(tty);
  	if (ld) {
  		if (ld->ops->flush_buffer)
  			ld->ops->flush_buffer(tty);
  		tty_ldisc_deref(ld);
  	}
  	tty_buffer_flush(tty);
  }
  EXPORT_SYMBOL_GPL(tty_ldisc_flush);
  
  /**
   *	tty_set_termios_ldisc		-	set ldisc field
   *	@tty: tty structure
   *	@num: line discipline number
   *
   *	This is probably overkill for real world processors but
   *	they are not on hot paths so a little discipline won't do
   *	any harm.
   *
   *	Locking: takes termios_mutex
   */
  
  static void tty_set_termios_ldisc(struct tty_struct *tty, int num)
  {
  	mutex_lock(&tty->termios_mutex);
  	tty->termios.c_line = num;
  	mutex_unlock(&tty->termios_mutex);
  }
  
  /**
   *	tty_ldisc_open		-	open a line discipline
   *	@tty: tty we are opening the ldisc on
   *	@ld: discipline to open
   *
   *	A helper opening method. Also a convenient debugging and check
   *	point.
   *
   *	Locking: always called with BTM already held.
   */
  
  static int tty_ldisc_open(struct tty_struct *tty, struct tty_ldisc *ld)
  {
  	WARN_ON(test_and_set_bit(TTY_LDISC_OPEN, &tty->flags));
  	if (ld->ops->open) {
  		int ret;
                  /* BTM here locks versus a hangup event */
  		ret = ld->ops->open(tty);
  		if (ret)
  			clear_bit(TTY_LDISC_OPEN, &tty->flags);
  		return ret;
  	}
  	return 0;
  }
  
  /**
   *	tty_ldisc_close		-	close a line discipline
   *	@tty: tty we are opening the ldisc on
   *	@ld: discipline to close
   *
   *	A helper close method. Also a convenient debugging and check
   *	point.
   */
  
  static void tty_ldisc_close(struct tty_struct *tty, struct tty_ldisc *ld)
  {
  	WARN_ON(!test_bit(TTY_LDISC_OPEN, &tty->flags));
  	clear_bit(TTY_LDISC_OPEN, &tty->flags);
  	if (ld->ops->close)
  		ld->ops->close(tty);
  }
  
  /**
   *	tty_ldisc_restore	-	helper for tty ldisc change
   *	@tty: tty to recover
   *	@old: previous ldisc
   *
   *	Restore the previous line discipline or N_TTY when a line discipline
   *	change fails due to an open error
   */
  
  static void tty_ldisc_restore(struct tty_struct *tty, struct tty_ldisc *old)
  {
  	char buf[64];
  	struct tty_ldisc *new_ldisc;
  	int r;
  
  	/* There is an outstanding reference here so this is safe */
  	old = tty_ldisc_get(tty, old->ops->num);
  	WARN_ON(IS_ERR(old));
  	tty->ldisc = old;
  	tty_set_termios_ldisc(tty, old->ops->num);
  	if (tty_ldisc_open(tty, old) < 0) {
  		tty_ldisc_put(old);
  		/* This driver is always present */
  		new_ldisc = tty_ldisc_get(tty, N_TTY);
  		if (IS_ERR(new_ldisc))
  			panic("n_tty: get");
  		tty->ldisc = new_ldisc;
  		tty_set_termios_ldisc(tty, N_TTY);
  		r = tty_ldisc_open(tty, new_ldisc);
  		if (r < 0)
  			panic("Couldn't open N_TTY ldisc for "
  			      "%s --- error %d.",
  			      tty_name(tty, buf), r);
  	}
  }
  
  /**
   *	tty_set_ldisc		-	set line discipline
   *	@tty: the terminal to set
   *	@ldisc: the line discipline
   *
   *	Set the discipline of a tty line. Must be called from a process
   *	context. The ldisc change logic has to protect itself against any
   *	overlapping ldisc change (including on the other end of pty pairs),
   *	the close of one side of a tty/pty pair, and eventually hangup.
   */
  
  int tty_set_ldisc(struct tty_struct *tty, int ldisc)
  {
  	int retval;
  	struct tty_ldisc *o_ldisc, *new_ldisc;
  	struct tty_struct *o_tty = tty->link;
  
  	new_ldisc = tty_ldisc_get(tty, ldisc);
  	if (IS_ERR(new_ldisc))
  		return PTR_ERR(new_ldisc);
  
  	retval = tty_ldisc_lock_pair_timeout(tty, o_tty, 5 * HZ);
  	if (retval) {
  		tty_ldisc_put(new_ldisc);
  		return retval;
  	}
  
  	/*
  	 *	Check the no-op case
  	 */
  
  	if (tty->ldisc->ops->num == ldisc) {
  		tty_ldisc_enable_pair(tty, o_tty);
  		tty_ldisc_put(new_ldisc);
  		return 0;
  	}
  
  	/* FIXME: why 'shutoff' input if the ldisc is locked? */
  	tty->receive_room = 0;
  
  	o_ldisc = tty->ldisc;
  	tty_lock(tty);
  
  	/* FIXME: for testing only */
  	WARN_ON(test_bit(TTY_HUPPED, &tty->flags));
  
  	if (test_bit(TTY_HUPPING, &tty->flags)) {
  		/* We were raced by the hangup method. It will have stomped
  		   the ldisc data and closed the ldisc down */
  		tty_ldisc_enable_pair(tty, o_tty);
  		tty_ldisc_put(new_ldisc);
  		tty_unlock(tty);
  		return -EIO;
  	}
  
  	/* Shutdown the current discipline. */
  	tty_ldisc_close(tty, o_ldisc);
  
  	/* Now set up the new line discipline. */
  	tty->ldisc = new_ldisc;
  	tty_set_termios_ldisc(tty, ldisc);
  
  	retval = tty_ldisc_open(tty, new_ldisc);
  	if (retval < 0) {
  		/* Back to the old one or N_TTY if we can't */
  		tty_ldisc_put(new_ldisc);
  		tty_ldisc_restore(tty, o_ldisc);
  	}
  
  	/* At this point we hold a reference to the new ldisc and a
  	   a reference to the old ldisc. If we ended up flipping back
  	   to the existing ldisc we have two references to it */
  
  	if (tty->ldisc->ops->num != o_ldisc->ops->num && tty->ops->set_ldisc)
  		tty->ops->set_ldisc(tty);
  
  	tty_ldisc_put(o_ldisc);
  
  	/*
  	 *	Allow ldisc referencing to occur again
  	 */
  	tty_ldisc_enable_pair(tty, o_tty);
  
  	/* Restart the work queue in case no characters kick it off. Safe if
  	   already running */
  	schedule_work(&tty->port->buf.work);
  	if (o_tty)
  		schedule_work(&o_tty->port->buf.work);
  
  	tty_unlock(tty);
  	return retval;
  }
  
  /**
   *	tty_reset_termios	-	reset terminal state
   *	@tty: tty to reset
   *
   *	Restore a terminal to the driver default state.
   */
  
  static void tty_reset_termios(struct tty_struct *tty)
  {
  	mutex_lock(&tty->termios_mutex);
  	tty->termios = tty->driver->init_termios;
  	tty->termios.c_ispeed = tty_termios_input_baud_rate(&tty->termios);
  	tty->termios.c_ospeed = tty_termios_baud_rate(&tty->termios);
  	mutex_unlock(&tty->termios_mutex);
  }
  
  
  /**
   *	tty_ldisc_reinit	-	reinitialise the tty ldisc
   *	@tty: tty to reinit
   *	@ldisc: line discipline to reinitialize
   *
   *	Switch the tty to a line discipline and leave the ldisc
   *	state closed
   */
  
  static int tty_ldisc_reinit(struct tty_struct *tty, int ldisc)
  {
  	struct tty_ldisc *ld = tty_ldisc_get(tty, ldisc);
  
  	if (IS_ERR(ld))
  		return -1;
  
  	tty_ldisc_close(tty, tty->ldisc);
  	tty_ldisc_put(tty->ldisc);
  	/*
  	 *	Switch the line discipline back
  	 */
  	tty->ldisc = ld;
  	tty_set_termios_ldisc(tty, ldisc);
  
  	return 0;
  }
  
  /**
   *	tty_ldisc_hangup		-	hangup ldisc reset
   *	@tty: tty being hung up
   *
   *	Some tty devices reset their termios when they receive a hangup
   *	event. In that situation we must also switch back to N_TTY properly
   *	before we reset the termios data.
   *
   *	Locking: We can take the ldisc mutex as the rest of the code is
   *	careful to allow for this.
   *
   *	In the pty pair case this occurs in the close() path of the
   *	tty itself so we must be careful about locking rules.
   */
  
  void tty_ldisc_hangup(struct tty_struct *tty)
  {
  	struct tty_ldisc *ld;
  	int reset = tty->driver->flags & TTY_DRIVER_RESET_TERMIOS;
  	int err = 0;
  
  	tty_ldisc_debug(tty, "closing ldisc: %p\n", tty->ldisc);
  
  	ld = tty_ldisc_ref(tty);
  	if (ld != NULL) {
  		if (ld->ops->flush_buffer)
  			ld->ops->flush_buffer(tty);
  		tty_driver_flush_buffer(tty);
  		if ((test_bit(TTY_DO_WRITE_WAKEUP, &tty->flags)) &&
  		    ld->ops->write_wakeup)
  			ld->ops->write_wakeup(tty);
  		if (ld->ops->hangup)
  			ld->ops->hangup(tty);
  		tty_ldisc_deref(ld);
  	}
  
  	wake_up_interruptible_poll(&tty->write_wait, POLLOUT);
  	wake_up_interruptible_poll(&tty->read_wait, POLLIN);
  
  	tty_unlock(tty);
  
  	/*
  	 * Shutdown the current line discipline, and reset it to
  	 * N_TTY if need be.
  	 *
  	 * Avoid racing set_ldisc or tty_ldisc_release
  	 */
  	tty_ldisc_lock_pair(tty, tty->link);
  	tty_lock(tty);
  
  	if (tty->ldisc) {
  
  		/* At this point we have a halted ldisc; we want to close it and
  		   reopen a new ldisc. We could defer the reopen to the next
  		   open but it means auditing a lot of other paths so this is
  		   a FIXME */
  		if (reset == 0) {
  
  			if (!tty_ldisc_reinit(tty, tty->termios.c_line))
  				err = tty_ldisc_open(tty, tty->ldisc);
  			else
  				err = 1;
  		}
  		/* If the re-open fails or we reset then go to N_TTY. The
  		   N_TTY open cannot fail */
  		if (reset || err) {
  			BUG_ON(tty_ldisc_reinit(tty, N_TTY));
  			WARN_ON(tty_ldisc_open(tty, tty->ldisc));
  		}
  	}
  	tty_ldisc_enable_pair(tty, tty->link);
  	if (reset)
  		tty_reset_termios(tty);
  
  	tty_ldisc_debug(tty, "re-opened ldisc: %p\n", tty->ldisc);
  }
  
  /**
   *	tty_ldisc_setup			-	open line discipline
   *	@tty: tty being shut down
   *	@o_tty: pair tty for pty/tty pairs
   *
   *	Called during the initial open of a tty/pty pair in order to set up the
   *	line disciplines and bind them to the tty. This has no locking issues
   *	as the device isn't yet active.
   */
  
  int tty_ldisc_setup(struct tty_struct *tty, struct tty_struct *o_tty)
  {
  	struct tty_ldisc *ld = tty->ldisc;
  	int retval;
  
  	retval = tty_ldisc_open(tty, ld);
  	if (retval)
  		return retval;
  
  	if (o_tty) {
  		retval = tty_ldisc_open(o_tty, o_tty->ldisc);
  		if (retval) {
  			tty_ldisc_close(tty, ld);
  			return retval;
  		}
  	}
  	return 0;
  }
  
  static void tty_ldisc_kill(struct tty_struct *tty)
  {
  	/*
  	 * Now kill off the ldisc
  	 */
  	tty_ldisc_close(tty, tty->ldisc);
  	tty_ldisc_put(tty->ldisc);
  	/* Force an oops if we mess this up */
  	tty->ldisc = NULL;
  
  	/* Ensure the next open requests the N_TTY ldisc */
  	tty_set_termios_ldisc(tty, N_TTY);
  }
  
  /**
   *	tty_ldisc_release		-	release line discipline
   *	@tty: tty being shut down
   *	@o_tty: pair tty for pty/tty pairs
   *
   *	Called during the final close of a tty/pty pair in order to shut down
   *	the line discpline layer. On exit the ldisc assigned is N_TTY and the
   *	ldisc has not been opened.
   */
  
  void tty_ldisc_release(struct tty_struct *tty, struct tty_struct *o_tty)
  {
  	/*
  	 * Shutdown this line discipline. As this is the final close,
  	 * it does not race with the set_ldisc code path.
  	 */
  
  	tty_ldisc_debug(tty, "closing ldisc: %p\n", tty->ldisc);
  
  	tty_ldisc_lock_pair(tty, o_tty);
  	tty_lock_pair(tty, o_tty);
  
  	tty_ldisc_kill(tty);
  	if (o_tty)
  		tty_ldisc_kill(o_tty);
  
  	tty_unlock_pair(tty, o_tty);
  	tty_ldisc_unlock_pair(tty, o_tty);
  
  	/* And the memory resources remaining (buffers, termios) will be
  	   disposed of when the kref hits zero */
  
  	tty_ldisc_debug(tty, "ldisc closed\n");
  }
  
  /**
   *	tty_ldisc_init		-	ldisc setup for new tty
   *	@tty: tty being allocated
   *
   *	Set up the line discipline objects for a newly allocated tty. Note that
   *	the tty structure is not completely set up when this call is made.
   */
  
  void tty_ldisc_init(struct tty_struct *tty)
  {
  	struct tty_ldisc *ld = tty_ldisc_get(tty, N_TTY);
  	if (IS_ERR(ld))
  		panic("n_tty: init_tty");
  	tty->ldisc = ld;
  }
  
  /**
   *	tty_ldisc_init		-	ldisc cleanup for new tty
   *	@tty: tty that was allocated recently
   *
   *	The tty structure must not becompletely set up (tty_ldisc_setup) when
   *      this call is made.
   */
  void tty_ldisc_deinit(struct tty_struct *tty)
  {
  	tty_ldisc_put(tty->ldisc);
  	tty->ldisc = NULL;
  }
  
  void tty_ldisc_begin(void)
  {
  	/* Setup the default TTY line discipline. */
  	(void) tty_register_ldisc(N_TTY, &tty_ldisc_N_TTY);
  }
  