Linux 4.16.11

[linux/fpc-iii.git] / drivers / tty / tty_ldisc.c

// SPDX-License-Identifier: GPL-2.0
#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/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...)        tty_debug(tty, f, ##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: -EINVAL if the discipline index is not [N_TTY..NR_LDISCS] or
 *                       if the discipline is not registered
 *               -EAGAIN if request_module() failed to load or register the
 *                       the discipline
 *               -ENOMEM if allocation failure
 *
 *               Otherwise, returns a pointer to the discipline and bumps the
 *               ref count
 *
 *      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);
        }

        /*
         * There is no way to handle allocation failure of only 16 bytes.
         * Let's simplify error handling and save more memory.
         */
        ld = kmalloc(sizeof(struct tty_ldisc), GFP_KERNEL | __GFP_NOFAIL);
        ld->ops = ldops;
        ld->tty = tty;

        return ld;
}

/**
 *      tty_ldisc_put           -       release the ldisc
 *
 *      Complement of tty_ldisc_get().
 */
static 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.
 *
 *      Returns: NULL if the tty has been hungup and not re-opened with
 *               a new file descriptor, otherwise valid ldisc reference
 *
 *      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: a file_operations routine (read/poll/write) should use this
 *      function to wait for any ldisc lifetime events to finish.
 */

struct tty_ldisc *tty_ldisc_ref_wait(struct tty_struct *tty)
{
        struct tty_ldisc *ld;

        ldsem_down_read(&tty->ldisc_sem, MAX_SCHEDULE_TIMEOUT);
        ld = tty->ldisc;
        if (!ld)
                ldsem_up_read(&tty->ldisc_sem);
        return ld;
}
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
__tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout)
{
        return ldsem_down_write(&tty->ldisc_sem, timeout);
}

static inline int
__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)
{
        ldsem_up_write(&tty->ldisc_sem);
}

int tty_ldisc_lock(struct tty_struct *tty, unsigned long timeout)
{
        int ret;

        ret = __tty_ldisc_lock(tty, timeout);
        if (!ret)
                return -EBUSY;
        set_bit(TTY_LDISC_HALTED, &tty->flags);
        return 0;
}

void tty_ldisc_unlock(struct tty_struct *tty)
{
        clear_bit(TTY_LDISC_HALTED, &tty->flags);
        __tty_ldisc_unlock(tty);
}

static int
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 tty_ldisc_lock_pair(struct tty_struct *tty, struct tty_struct *tty2)
{
        tty_ldisc_lock_pair_timeout(tty, tty2, MAX_SCHEDULE_TIMEOUT);
}

static void tty_ldisc_unlock_pair(struct tty_struct *tty,
                                  struct tty_struct *tty2)
{
        __tty_ldisc_unlock(tty);
        if (tty2)
                __tty_ldisc_unlock(tty2);
}

/**
 *      tty_ldisc_flush -       flush line discipline queue
 *      @tty: tty
 *
 *      Flush the line discipline queue (if any) and the tty flip buffers
 *      for this tty.
 */

void tty_ldisc_flush(struct tty_struct *tty)
{
        struct tty_ldisc *ld = tty_ldisc_ref(tty);

        tty_buffer_flush(tty, ld);
        if (ld)
                tty_ldisc_deref(ld);
}
EXPORT_SYMBOL_GPL(tty_ldisc_flush);

/**
 *      tty_set_termios_ldisc           -       set ldisc field
 *      @tty: tty structure
 *      @disc: 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.
 *
 *      The line discipline-related tty_struct fields are reset to
 *      prevent the ldisc driver from re-using stale information for
 *      the new ldisc instance.
 *
 *      Locking: takes termios_rwsem
 */

static void tty_set_termios_ldisc(struct tty_struct *tty, int disc)
{
        down_write(&tty->termios_rwsem);
        tty->termios.c_line = disc;
        up_write(&tty->termios_rwsem);

        tty->disc_data = NULL;
        tty->receive_room = 0;
}

/**
 *      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);

                tty_ldisc_debug(tty, "%p: opened\n", ld);
                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_debug(tty, "%p: closed\n", ld);
}

/**
 *      tty_ldisc_failto        -       helper for ldisc failback
 *      @tty: tty to open the ldisc on
 *      @ld: ldisc we are trying to fail back to
 *
 *      Helper to try and recover a tty when switching back to the old
 *      ldisc fails and we need something attached.
 */

static int tty_ldisc_failto(struct tty_struct *tty, int ld)
{
        struct tty_ldisc *disc = tty_ldisc_get(tty, ld);
        int r;

        if (IS_ERR(disc))
                return PTR_ERR(disc);
        tty->ldisc = disc;
        tty_set_termios_ldisc(tty, ld);
        if ((r = tty_ldisc_open(tty, disc)) < 0)
                tty_ldisc_put(disc);
        return r;
}

/**
 *      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)
{
        /* There is an outstanding reference here so this is safe */
        if (tty_ldisc_failto(tty, old->ops->num) < 0) {
                const char *name = tty_name(tty);

                pr_warn("Falling back ldisc for %s.\n", name);
                /* The traditional behaviour is to fall back to N_TTY, we
                   want to avoid falling back to N_NULL unless we have no
                   choice to avoid the risk of breaking anything */
                if (tty_ldisc_failto(tty, N_TTY) < 0 &&
                    tty_ldisc_failto(tty, N_NULL) < 0)
                        panic("Couldn't open N_NULL ldisc for %s.", name);
        }
}

/**
 *      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 disc)
{
        int retval;
        struct tty_ldisc *old_ldisc, *new_ldisc;

        new_ldisc = tty_ldisc_get(tty, disc);
        if (IS_ERR(new_ldisc))
                return PTR_ERR(new_ldisc);

        tty_lock(tty);
        retval = tty_ldisc_lock(tty, 5 * HZ);
        if (retval)
                goto err;

        if (!tty->ldisc) {
                retval = -EIO;
                goto out;
        }

        /* Check the no-op case */
        if (tty->ldisc->ops->num == disc)
                goto out;

        if (test_bit(TTY_HUPPED, &tty->flags)) {
                /* We were raced by hangup */
                retval = -EIO;
                goto out;
        }

        old_ldisc = tty->ldisc;

        /* Shutdown the old discipline. */
        tty_ldisc_close(tty, old_ldisc);

        /* Now set up the new line discipline. */
        tty->ldisc = new_ldisc;
        tty_set_termios_ldisc(tty, disc);

        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, old_ldisc);
        }

        if (tty->ldisc->ops->num != old_ldisc->ops->num && tty->ops->set_ldisc) {
                down_read(&tty->termios_rwsem);
                tty->ops->set_ldisc(tty);
                up_read(&tty->termios_rwsem);
        }

        /* At this point we hold a reference to the new ldisc and a
           reference to the old ldisc, or we hold two references to
           the old ldisc (if it was restored as part of error cleanup
           above). In either case, releasing a single reference from
           the old ldisc is correct. */
        new_ldisc = old_ldisc;
out:
        tty_ldisc_unlock(tty);

        /* Restart the work queue in case no characters kick it off. Safe if
           already running */
        tty_buffer_restart_work(tty->port);
err:
        tty_ldisc_put(new_ldisc);       /* drop the extra reference */
        tty_unlock(tty);
        return retval;
}
EXPORT_SYMBOL_GPL(tty_set_ldisc);

/**
 *      tty_ldisc_kill  -       teardown ldisc
 *      @tty: tty being released
 *
 *      Perform final close of the ldisc and reset tty->ldisc
 */
static void tty_ldisc_kill(struct tty_struct *tty)
{
        if (!tty->ldisc)
                return;
        /*
         * 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;
}

/**
 *      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)
{
        down_write(&tty->termios_rwsem);
        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);
        up_write(&tty->termios_rwsem);
}


/**
 *      tty_ldisc_reinit        -       reinitialise the tty ldisc
 *      @tty: tty to reinit
 *      @disc: line discipline to reinitialize
 *
 *      Completely reinitialize the line discipline state, by closing the
 *      current instance, if there is one, and opening a new instance. If
 *      an error occurs opening the new non-N_TTY instance, the instance
 *      is dropped and tty->ldisc reset to NULL. The caller can then retry
 *      with N_TTY instead.
 *
 *      Returns 0 if successful, otherwise error code < 0
 */

int tty_ldisc_reinit(struct tty_struct *tty, int disc)
{
        struct tty_ldisc *ld;
        int retval;

        ld = tty_ldisc_get(tty, disc);
        if (IS_ERR(ld)) {
                BUG_ON(disc == N_TTY);
                return PTR_ERR(ld);
        }

        if (tty->ldisc) {
                tty_ldisc_close(tty, tty->ldisc);
                tty_ldisc_put(tty->ldisc);
        }

        /* switch the line discipline */
        tty->ldisc = ld;
        tty_set_termios_ldisc(tty, disc);
        retval = tty_ldisc_open(tty, tty->ldisc);
        if (retval) {
                tty_ldisc_put(tty->ldisc);
                tty->ldisc = NULL;
        }
        return retval;
}

/**
 *      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, bool reinit)
{
        struct tty_ldisc *ld;

        tty_ldisc_debug(tty, "%p: hangup\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, EPOLLOUT);
        wake_up_interruptible_poll(&tty->read_wait, EPOLLIN);

        /*
         * 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(tty, MAX_SCHEDULE_TIMEOUT);

        if (tty->driver->flags & TTY_DRIVER_RESET_TERMIOS)
                tty_reset_termios(tty);

        if (tty->ldisc) {
                if (reinit) {
                        if (tty_ldisc_reinit(tty, tty->termios.c_line) < 0 &&
                            tty_ldisc_reinit(tty, N_TTY) < 0)
                                WARN_ON(tty_ldisc_reinit(tty, N_NULL) < 0);
                } else
                        tty_ldisc_kill(tty);
        }
        tty_ldisc_unlock(tty);
}

/**
 *      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)
{
        int retval = tty_ldisc_open(tty, tty->ldisc);
        if (retval)
                return retval;

        if (o_tty) {
                retval = tty_ldisc_open(o_tty, o_tty->ldisc);
                if (retval) {
                        tty_ldisc_close(tty, tty->ldisc);
                        return retval;
                }
        }
        return 0;
}

/**
 *      tty_ldisc_release               -       release line discipline
 *      @tty: tty being shut down (or one end of pty pair)
 *
 *      Called during the final close of a tty or a pty pair in order to shut
 *      down the line discpline layer. On exit, each tty's ldisc is NULL.
 */

void tty_ldisc_release(struct tty_struct *tty)
{
        struct tty_struct *o_tty = tty->link;

        /*
         * Shutdown this line discipline. As this is the final close,
         * it does not race with the set_ldisc code path.
         */

        tty_ldisc_lock_pair(tty, o_tty);
        tty_ldisc_kill(tty);
        if (o_tty)
                tty_ldisc_kill(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, "released\n");
}
EXPORT_SYMBOL_GPL(tty_ldisc_release);

/**
 *      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.
 */

int tty_ldisc_init(struct tty_struct *tty)
{
        struct tty_ldisc *ld = tty_ldisc_get(tty, N_TTY);
        if (IS_ERR(ld))
                return PTR_ERR(ld);
        tty->ldisc = ld;
        return 0;
}

/**
 *      tty_ldisc_deinit        -       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)
{
        if (tty->ldisc)
                tty_ldisc_put(tty->ldisc);
        tty->ldisc = NULL;
}