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    Subject[PATCH] Update termios to use per tty semaphore (backport from 2.6.x)
    This is a 2.4.x backport of the termios locking changes made in 2.6.x:

    # 2004/10/02 15:46:35-07:00 alan@lxorguk.ukuu.org.uk
    # [PATCH] Update termios to use per tty semaphore
    #
    # This makes the agreed change of termios locking to be semaphore based
    # sleep locking. This is needed for USB in particular as it has to use
    # messaging to issue terminal mode changes.
    #
    # This code passes Torvalds test grades 0, 1 and 2 (it looks ok, it
    # compiles and it booted). It does mean that a driver cannot take an
    # atomic peek at termios data during an interrupt. Nobody seems to be
    # doing this although some of the driver receive paths for line
    # disciplines will eventually want to (n_tty currently doesn't do this
    # locked on the receive path). Since the ldisc is given a chance to copy
    # any essential bits on the ->set_termios path this seems not to be a
    # problem.

    --- kernel-source-2.4.27/include/linux/tty.h.termios 2004-10-31 18:07:10 +0300
    +++ kernel-source-2.4.27/include/linux/tty.h 2004-10-31 20:23:38 +0300
    @@ -261,6 +261,7 @@ struct tty_struct {
    int magic;
    struct tty_driver driver;
    struct tty_ldisc ldisc;
    + struct semaphore termios_sem;
    struct termios *termios, *termios_locked;
    int pgrp;
    int session;
    --- kernel-source-2.4.27/drivers/char/tty_io.c.termios 2004-10-31 18:04:37 +0300
    +++ kernel-source-2.4.27/drivers/char/tty_io.c 2004-10-31 20:26:13 +0300
    @@ -118,8 +118,6 @@ extern void disable_early_printk(void);
    #define TTY_PARANOIA_CHECK 1
    #define CHECK_TTY_COUNT 1

    -/* Lock for tty_termios changes - private to tty_io/tty_ioctl */
    -spinlock_t tty_termios_lock = SPIN_LOCK_UNLOCKED;
    struct termios tty_std_termios; /* for the benefit of tty drivers */
    struct tty_driver *tty_drivers; /* linked list of tty drivers */

    @@ -269,10 +267,9 @@ static int check_tty_count(struct tty_st

    static void tty_set_termios_ldisc(struct tty_struct *tty, int num)
    {
    - unsigned long flags;
    - spin_lock_irqsave(&tty_termios_lock, flags);
    + down(&tty->termios_sem);
    tty->termios->c_line = num;
    - spin_unlock_irqrestore(&tty_termios_lock, flags);
    + up(&tty->termios_sem);
    }

    /*
    @@ -803,10 +800,9 @@ void do_tty_hangup(void *data)

    if (tty->driver.flags & TTY_DRIVER_RESET_TERMIOS)
    {
    - unsigned long flags;
    - spin_lock_irqsave(&tty_termios_lock, flags);
    + down(&tty->termios_sem);
    *tty->termios = tty->driver.init_termios;
    - spin_unlock_irqrestore(&tty_termios_lock, flags);
    + up(&tty->termios_sem);
    }

    /* Defer ldisc switch */
    @@ -2452,6 +2448,7 @@ static void initialize_tty_struct(struct
    tty->flip.tqueue.routine = flush_to_ldisc;
    tty->flip.tqueue.data = tty;
    init_MUTEX(&tty->flip.pty_sem);
    + init_MUTEX(&tty->termios_sem);
    init_waitqueue_head(&tty->write_wait);
    init_waitqueue_head(&tty->read_wait);
    tty->tq_hangup.routine = do_tty_hangup;
    --- kernel-source-2.4.27/drivers/char/tty_ioctl.c.termios 2004-10-31 18:04:37 +0300
    +++ kernel-source-2.4.27/drivers/char/tty_ioctl.c 2004-10-31 20:31:47 +0300
    @@ -29,8 +29,6 @@

    #undef DEBUG

    -extern spinlock_t tty_termios_lock;
    -
    /*
    * Internal flag options for termios setting behavior
    */
    @@ -99,7 +97,6 @@ static void change_termios(struct tty_st
    int canon_change;
    struct termios old_termios = *tty->termios;
    struct tty_ldisc *ld;
    - unsigned long flags;

    /*
    * Perform the actual termios internal changes under lock.
    @@ -107,7 +104,7 @@ static void change_termios(struct tty_st

    /* FIXME: we need to decide on some locking/ordering semantics
    for the set_termios notification eventually */
    - spin_lock_irqsave(&tty_termios_lock, flags);
    + down(&tty->termios_sem);

    *tty->termios = *new_termios;
    unset_locked_termios(tty->termios, &old_termios, tty->termios_locked);
    @@ -147,8 +144,7 @@ static void change_termios(struct tty_st
    (ld->set_termios)(tty, &old_termios);
    tty_ldisc_deref(ld);
    }
    - spin_unlock_irqrestore(&tty_termios_lock, flags);
    -
    + up(&tty->termios_sem);
    }

    static int set_termios(struct tty_struct * tty, unsigned long arg, int opt)
    @@ -244,13 +240,13 @@ static int get_sgttyb(struct tty_struct
    struct sgttyb tmp;
    unsigned long flags;

    - spin_lock_irqsave(&tty_termios_lock, flags);
    + down(&tty->termios_sem);
    tmp.sg_ispeed = 0;
    tmp.sg_ospeed = 0;
    tmp.sg_erase = tty->termios->c_cc[VERASE];
    tmp.sg_kill = tty->termios->c_cc[VKILL];
    tmp.sg_flags = get_sgflags(tty);
    - spin_unlock_irqrestore(&tty_termios_lock, flags);
    + up(&tty->termios_sem);

    return copy_to_user(sgttyb, &tmp, sizeof(tmp)) ? -EFAULT : 0;
    }
    @@ -286,19 +282,18 @@ static int set_sgttyb(struct tty_struct
    int retval;
    struct sgttyb tmp;
    struct termios termios;
    - unsigned long flags;

    retval = tty_check_change(tty);
    if (retval)
    return retval;
    if (copy_from_user(&tmp, sgttyb, sizeof(tmp)))
    return -EFAULT;
    - spin_lock_irqsave(&tty_termios_lock, flags);
    + down(&tty->termios_sem);
    termios = *tty->termios;
    termios.c_cc[VERASE] = tmp.sg_erase;
    termios.c_cc[VKILL] = tmp.sg_kill;
    set_sgflags(&termios, tmp.sg_flags);
    - spin_unlock_irqrestore(&tty_termios_lock, flags);
    + up(&tty->termios_sem);
    change_termios(tty, &termios);
    return 0;
    }
    @@ -533,11 +528,11 @@ int n_tty_ioctl(struct tty_struct * tty,
    case TIOCSSOFTCAR:
    if (get_user(arg, (unsigned int *) arg))
    return -EFAULT;
    - spin_lock_irqsave(&tty_termios_lock, flags);
    + down(&tty->termios_sem);
    tty->termios->c_cflag =
    ((tty->termios->c_cflag & ~CLOCAL) |
    (arg ? CLOCAL : 0));
    - spin_unlock_irqrestore(&tty_termios_lock, flags);
    + up(&tty->termios_sem);
    return 0;
    default:
    return -ENOIOCTLCMD;
    --- kernel-source-2.4.27/Documentation/tty.txt.termios 2004-10-31 18:04:36 +0300
    +++ kernel-source-2.4.27/Documentation/tty.txt 2004-10-31 20:23:38 +0300
    @@ -158,8 +158,8 @@ write_room() - Return the number of char

    ioctl() - Called when an ioctl may be for the driver

    -set_termios() - Called on termios change, may get parallel calls,
    - may block for now (may change that)
    +set_termios() - Called on termios change, serialized against
    + itself by a semaphore. May sleep.

    set_ldisc() - Notifier for discipline change. At the point this
    is done the discipline is not yet usable. Can now[unhandled content-type:application/pgp-signature]
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