[PATCH] Update termios to use per tty semaphore (backport from 2.6.x)

From: Sergey Vlasov
Date: Sun Nov 07 2004 - 15:12:12 EST

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@xxxxxxxxxxxxxxxxxxx
# [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

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