bug: introduce BUILD_BUG_ON_INVALID() macro

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

#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>

/*
 *      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_SPINLOCK(tty_ldisc_lock);
static DECLARE_WAIT_QUEUE_HEAD(tty_ldisc_wait);
/* Line disc dispatch table */
static struct tty_ldisc_ops *tty_ldiscs[NR_LDISCS];

static inline struct tty_ldisc *get_ldisc(struct tty_ldisc *ld)
{
        if (ld)
                atomic_inc(&ld->users);
        return ld;
}

static void put_ldisc(struct tty_ldisc *ld)
{
        unsigned long flags;

        if (WARN_ON_ONCE(!ld))
                return;

        /*
         * If this is the last user, free the ldisc, and
         * release the ldisc ops.
         *
         * We really want an "atomic_dec_and_lock_irqsave()",
         * but we don't have it, so this does it by hand.
         */
        local_irq_save(flags);
        if (atomic_dec_and_lock(&ld->users, &tty_ldisc_lock)) {
                struct tty_ldisc_ops *ldo = ld->ops;

                ldo->refcount--;
                module_put(ldo->owner);
                spin_unlock_irqrestore(&tty_ldisc_lock, flags);

                kfree(ld);
                return;
        }
        local_irq_restore(flags);
        wake_up(&ld->wq_idle);
}

/**
 *      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_ldisc_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;

        spin_lock_irqsave(&tty_ldisc_lock, flags);
        tty_ldiscs[disc] = new_ldisc;
        new_ldisc->num = disc;
        new_ldisc->refcount = 0;
        spin_unlock_irqrestore(&tty_ldisc_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_ldisc_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;

        spin_lock_irqsave(&tty_ldisc_lock, flags);
        if (tty_ldiscs[disc]->refcount)
                ret = -EBUSY;
        else
                tty_ldiscs[disc] = NULL;
        spin_unlock_irqrestore(&tty_ldisc_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;

        spin_lock_irqsave(&tty_ldisc_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;
                }
        }
        spin_unlock_irqrestore(&tty_ldisc_lock, flags);
        return ret;
}

static void put_ldops(struct tty_ldisc_ops *ldops)
{
        unsigned long flags;

        spin_lock_irqsave(&tty_ldisc_lock, flags);
        ldops->refcount--;
        module_put(ldops->owner);
        spin_unlock_irqrestore(&tty_ldisc_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_ldisc_lock to guard against ldisc races
 */

static struct tty_ldisc *tty_ldisc_get(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;
        atomic_set(&ld->users, 1);
        init_waitqueue_head(&ld->wq_idle);

        return 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_assign        -       set ldisc on a tty
 *      @tty: tty to assign
 *      @ld: line discipline
 *
 *      Install an instance of a line discipline into a tty structure. The
 *      ldisc must have a reference count above zero to ensure it remains.
 *      The tty instance refcount starts at zero.
 *
 *      Locking:
 *              Caller must hold references
 */

static void tty_ldisc_assign(struct tty_struct *tty, struct tty_ldisc *ld)
{
        tty->ldisc = ld;
}

/**
 *      tty_ldisc_try           -       internal helper
 *      @tty: the tty
 *
 *      Make a single attempt to grab and bump the refcount on
 *      the tty ldisc. Return 0 on failure or 1 on success. This is
 *      used to implement both the waiting and non waiting versions
 *      of tty_ldisc_ref
 *
 *      Locking: takes tty_ldisc_lock
 */

static struct tty_ldisc *tty_ldisc_try(struct tty_struct *tty)
{
        unsigned long flags;
        struct tty_ldisc *ld;

        spin_lock_irqsave(&tty_ldisc_lock, flags);
        ld = NULL;
        if (test_bit(TTY_LDISC, &tty->flags))
                ld = get_ldisc(tty->ldisc);
        spin_unlock_irqrestore(&tty_ldisc_lock, flags);
        return ld;
}

/**
 *      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)
 *
 *      Locking: call functions take tty_ldisc_lock
 */

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

        /* wait_event is a macro */
        wait_event(tty_ldisc_wait, (ld = tty_ldisc_try(tty)) != NULL);
        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.
 *
 *      Locking: called functions take tty_ldisc_lock
 */

struct tty_ldisc *tty_ldisc_ref(struct tty_struct *tty)
{
        return tty_ldisc_try(tty);
}
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.
 *
 *      Locking: takes tty_ldisc_lock
 */

void tty_ldisc_deref(struct tty_ldisc *ld)
{
        put_ldisc(ld);
}
EXPORT_SYMBOL_GPL(tty_ldisc_deref);

static inline void tty_ldisc_put(struct tty_ldisc *ld)
{
        put_ldisc(ld);
}

/**
 *      tty_ldisc_enable        -       allow ldisc use
 *      @tty: terminal to activate ldisc on
 *
 *      Set the TTY_LDISC flag when the line discipline can be called
 *      again. Do necessary wakeups for existing sleepers. Clear the LDISC
 *      changing flag to indicate any ldisc change is now over.
 *
 *      Note: nobody should set the TTY_LDISC bit except via this function.
 *      Clearing directly is allowed.
 */

void tty_ldisc_enable(struct tty_struct *tty)
{
        set_bit(TTY_LDISC, &tty->flags);
        clear_bit(TTY_LDISC_CHANGING, &tty->flags);
        wake_up(&tty_ldisc_wait);
}

/**
 *      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(old->ops->num);
        WARN_ON(IS_ERR(old));
        tty_ldisc_assign(tty, 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(N_TTY);
                if (IS_ERR(new_ldisc))
                        panic("n_tty: get");
                tty_ldisc_assign(tty, 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_ldisc_halt          -       shut down the line discipline
 *      @tty: tty device
 *
 *      Shut down the line discipline and work queue for this tty device.
 *      The TTY_LDISC flag being cleared ensures no further references can
 *      be obtained while the delayed work queue halt ensures that no more
 *      data is fed to the ldisc.
 *
 *      You need to do a 'flush_scheduled_work()' (outside the ldisc_mutex)
 *      in order to make sure any currently executing ldisc work is also
 *      flushed.
 */

static int tty_ldisc_halt(struct tty_struct *tty)
{
        clear_bit(TTY_LDISC, &tty->flags);
        return cancel_work_sync(&tty->buf.work);
}

/**
 *      tty_ldisc_flush_works   -       flush all works of a tty
 *      @tty: tty device to flush works for
 *
 *      Sync flush all works belonging to @tty.
 */
static void tty_ldisc_flush_works(struct tty_struct *tty)
{
        flush_work_sync(&tty->hangup_work);
        flush_work_sync(&tty->SAK_work);
        flush_work_sync(&tty->buf.work);
}

/**
 *      tty_ldisc_wait_idle     -       wait for the ldisc to become idle
 *      @tty: tty to wait for
 *      @timeout: for how long to wait at most
 *
 *      Wait for the line discipline to become idle. The discipline must
 *      have been halted for this to guarantee it remains idle.
 */
static int tty_ldisc_wait_idle(struct tty_struct *tty, long timeout)
{
        long ret;
        ret = wait_event_timeout(tty->ldisc->wq_idle,
                        atomic_read(&tty->ldisc->users) == 1, timeout);
        return ret > 0 ? 0 : -EBUSY;
}

/**
 *      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.
 *
 *      Locking: takes tty_ldisc_lock, termios_mutex
 */

int tty_set_ldisc(struct tty_struct *tty, int ldisc)
{
        int retval;
        struct tty_ldisc *o_ldisc, *new_ldisc;
        int work, o_work = 0;
        struct tty_struct *o_tty;

        new_ldisc = tty_ldisc_get(ldisc);
        if (IS_ERR(new_ldisc))
                return PTR_ERR(new_ldisc);

        tty_lock(tty);
        /*
         *      We need to look at the tty locking here for pty/tty pairs
         *      when both sides try to change in parallel.
         */

        o_tty = tty->link;      /* o_tty is the pty side or NULL */


        /*
         *      Check the no-op case
         */

        if (tty->ldisc->ops->num == ldisc) {
                tty_unlock(tty);
                tty_ldisc_put(new_ldisc);
                return 0;
        }

        tty_unlock(tty);
        /*
         *      Problem: What do we do if this blocks ?
         *      We could deadlock here
         */

        tty_wait_until_sent(tty, 0);

        tty_lock(tty);
        mutex_lock(&tty->ldisc_mutex);

        /*
         *      We could be midstream of another ldisc change which has
         *      dropped the lock during processing. If so we need to wait.
         */

        while (test_bit(TTY_LDISC_CHANGING, &tty->flags)) {
                mutex_unlock(&tty->ldisc_mutex);
                tty_unlock(tty);
                wait_event(tty_ldisc_wait,
                        test_bit(TTY_LDISC_CHANGING, &tty->flags) == 0);
                tty_lock(tty);
                mutex_lock(&tty->ldisc_mutex);
        }

        set_bit(TTY_LDISC_CHANGING, &tty->flags);

        /*
         *      No more input please, we are switching. The new ldisc
         *      will update this value in the ldisc open function
         */

        tty->receive_room = 0;

        o_ldisc = tty->ldisc;

        tty_unlock(tty);
        /*
         *      Make sure we don't change while someone holds a
         *      reference to the line discipline. The TTY_LDISC bit
         *      prevents anyone taking a reference once it is clear.
         *      We need the lock to avoid racing reference takers.
         *
         *      We must clear the TTY_LDISC bit here to avoid a livelock
         *      with a userspace app continually trying to use the tty in
         *      parallel to the change and re-referencing the tty.
         */

        work = tty_ldisc_halt(tty);
        if (o_tty)
                o_work = tty_ldisc_halt(o_tty);

        /*
         * Wait for ->hangup_work and ->buf.work handlers to terminate.
         * We must drop the mutex here in case a hangup is also in process.
         */

        mutex_unlock(&tty->ldisc_mutex);

        tty_ldisc_flush_works(tty);

        retval = tty_ldisc_wait_idle(tty, 5 * HZ);

        tty_lock(tty);
        mutex_lock(&tty->ldisc_mutex);

        /* handle wait idle failure locked */
        if (retval) {
                tty_ldisc_put(new_ldisc);
                goto enable;
        }

        if (test_bit(TTY_HUPPED, &tty->flags)) {
                /* We were raced by the hangup method. It will have stomped
                   the ldisc data and closed the ldisc down */
                clear_bit(TTY_LDISC_CHANGING, &tty->flags);
                mutex_unlock(&tty->ldisc_mutex);
                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_assign(tty, 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);

enable:
        /*
         *      Allow ldisc referencing to occur again
         */

        tty_ldisc_enable(tty);
        if (o_tty)
                tty_ldisc_enable(o_tty);

        /* Restart the work queue in case no characters kick it off. Safe if
           already running */
        if (work)
                schedule_work(&tty->buf.work);
        if (o_work)
                schedule_work(&o_tty->buf.work);
        mutex_unlock(&tty->ldisc_mutex);
        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(ldisc);

        if (IS_ERR(ld))
                return -1;

        tty_ldisc_close(tty, tty->ldisc);
        tty_ldisc_put(tty->ldisc);
        tty->ldisc = NULL;
        /*
         *      Switch the line discipline back
         */
        tty_ldisc_assign(tty, 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;

        /*
         * FIXME! What are the locking issues here? This may me overdoing
         * things... This question is especially important now that we've
         * removed the irqlock.
         */
        ld = tty_ldisc_ref(tty);
        if (ld != NULL) {
                /* We may have no line discipline at this point */
                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);
        }
        /*
         * FIXME: Once we trust the LDISC code better we can wait here for
         * ldisc completion and fix the driver call race
         */
        wake_up_interruptible_poll(&tty->write_wait, POLLOUT);
        wake_up_interruptible_poll(&tty->read_wait, POLLIN);
        /*
         * Shutdown the current line discipline, and reset it to
         * N_TTY if need be.
         *
         * Avoid racing set_ldisc or tty_ldisc_release
         */
        mutex_lock(&tty->ldisc_mutex);

        /*
         * this is like tty_ldisc_halt, but we need to give up
         * the BTM before calling cancel_work_sync, which may
         * need to wait for another function taking the BTM
         */
        clear_bit(TTY_LDISC, &tty->flags);
        tty_unlock(tty);
        cancel_work_sync(&tty->buf.work);
        mutex_unlock(&tty->ldisc_mutex);
retry:
        tty_lock(tty);
        mutex_lock(&tty->ldisc_mutex);

        /* At this point we have a closed ldisc and we want to
           reopen it. We could defer this to the next open but
           it means auditing a lot of other paths so this is
           a FIXME */
        if (tty->ldisc) {       /* Not yet closed */
                if (atomic_read(&tty->ldisc->users) != 1) {
                        char cur_n[TASK_COMM_LEN], tty_n[64];
                        long timeout = 3 * HZ;
                        tty_unlock(tty);

                        while (tty_ldisc_wait_idle(tty, timeout) == -EBUSY) {
                                timeout = MAX_SCHEDULE_TIMEOUT;
                                printk_ratelimited(KERN_WARNING
                                        "%s: waiting (%s) for %s took too long, but we keep waiting...\n",
                                        __func__, get_task_comm(cur_n, current),
                                        tty_name(tty, tty_n));
                        }
                        mutex_unlock(&tty->ldisc_mutex);
                        goto retry;
                }

                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(tty);
        }
        mutex_unlock(&tty->ldisc_mutex);
        if (reset)
                tty_reset_termios(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)
{
        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;
                }
                tty_ldisc_enable(o_tty);
        }
        tty_ldisc_enable(tty);
        return 0;
}

static void tty_ldisc_kill(struct tty_struct *tty)
{
        mutex_lock(&tty->ldisc_mutex);
        /*
         * 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);
        mutex_unlock(&tty->ldisc_mutex);
}

/**
 *      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)
{
        /*
         * Prevent flush_to_ldisc() from rescheduling the work for later.  Then
         * kill any delayed work. As this is the final close it does not
         * race with the set_ldisc code path.
         */

        tty_unlock_pair(tty, o_tty);
        tty_ldisc_halt(tty);
        tty_ldisc_flush_works(tty);
        if (o_tty) {
                tty_ldisc_halt(o_tty);
                tty_ldisc_flush_works(o_tty);
        }
        tty_lock_pair(tty, o_tty);


        tty_ldisc_kill(tty);
        if (o_tty)
                tty_ldisc_kill(o_tty);

        /* And the memory resources remaining (buffers, termios) will be
           disposed of when the kref hits zero */
}

/**
 *      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(N_TTY);
        if (IS_ERR(ld))
                panic("n_tty: init_tty");
        tty_ldisc_assign(tty, 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)
{
        put_ldisc(tty->ldisc);
        tty_ldisc_assign(tty, NULL);
}

void tty_ldisc_begin(void)
{
        /* Setup the default TTY line discipline. */
        (void) tty_register_ldisc(N_TTY, &tty_ldisc_N_TTY);
}