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tty: Serialize tty flow control changes with flow_lock 

Without serialization, the flow control state can become inverted
wrt. the actual hardware state. For example,

CPU 0                          | CPU 1
stop_tty()                     |
  lock ctrl_lock               |
  tty->stopped = 1             |
  unlock ctrl_lock             |
                               | start_tty()
                               |   lock ctrl_lock
                               |   tty->stopped = 0
                               |   unlock ctrl_lock
                               |   driver->start()
  driver->stop()               |

In this case, the flow control state now indicates the tty has
been started, but the actual hardware state has actually been stopped.

Introduce tty->flow_lock spinlock to serialize tty flow control changes.
Split out unlocked __start_tty()/__stop_tty() flavors for use by
ioctl(TCXONC) in follow-on patch.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Peter Hurley 2014-09-10 15:06:31 -04:00 committed by Greg Kroah-Hartman
parent d7a855bd6a
commit f9e053dcfc
3 changed files with 36 additions and 12 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

39
drivers/tty/tty_io.c
View File

@ -919,18 +919,18 @@ void no_tty(void)
* but not always.
*
* Locking:
* Uses the tty control lock internally
* ctrl_lock
* flow_lock
*/
void stop_tty(struct tty_struct *tty)
void __stop_tty(struct tty_struct *tty)
{
unsigned long flags;
spin_lock_irqsave(&tty->ctrl_lock, flags);
if (tty->stopped) {
spin_unlock_irqrestore(&tty->ctrl_lock, flags);
if (tty->stopped)
return;
}
tty->stopped = 1;
spin_lock_irqsave(&tty->ctrl_lock, flags);
if (tty->link && tty->link->packet) {
tty->ctrl_status &= ~TIOCPKT_START;
tty->ctrl_status |= TIOCPKT_STOP;
@ -941,6 +941,14 @@ void stop_tty(struct tty_struct *tty)
(tty->ops->stop)(tty);
}
void stop_tty(struct tty_struct *tty)
{
unsigned long flags;
spin_lock_irqsave(&tty->flow_lock, flags);
__stop_tty(tty);
spin_unlock_irqrestore(&tty->flow_lock, flags);
}
EXPORT_SYMBOL(stop_tty);
/**
@ -954,17 +962,17 @@ EXPORT_SYMBOL(stop_tty);
*
* Locking:
* ctrl_lock
* flow_lock
*/
void start_tty(struct tty_struct *tty)
void __start_tty(struct tty_struct *tty)
{
unsigned long flags;
spin_lock_irqsave(&tty->ctrl_lock, flags);
if (!tty->stopped || tty->flow_stopped) {
spin_unlock_irqrestore(&tty->ctrl_lock, flags);
if (!tty->stopped || tty->flow_stopped)
return;
}
tty->stopped = 0;
spin_lock_irqsave(&tty->ctrl_lock, flags);
if (tty->link && tty->link->packet) {
tty->ctrl_status &= ~TIOCPKT_STOP;
tty->ctrl_status |= TIOCPKT_START;
@ -977,6 +985,14 @@ void start_tty(struct tty_struct *tty)
tty_wakeup(tty);
}
void start_tty(struct tty_struct *tty)
{
unsigned long flags;
spin_lock_irqsave(&tty->flow_lock, flags);
__start_tty(tty);
spin_unlock_irqrestore(&tty->flow_lock, flags);
}
EXPORT_SYMBOL(start_tty);
/* We limit tty time update visibility to every 8 seconds or so. */
@ -3019,6 +3035,7 @@ struct tty_struct *alloc_tty_struct(struct tty_driver *driver, int idx)
INIT_WORK(&tty->hangup_work, do_tty_hangup);
mutex_init(&tty->atomic_write_lock);
spin_lock_init(&tty->ctrl_lock);
spin_lock_init(&tty->flow_lock);
INIT_LIST_HEAD(&tty->tty_files);
INIT_WORK(&tty->SAK_work, do_SAK_work);

5
include/linux/tty.h
View File

@ -252,6 +252,7 @@ struct tty_struct {
struct rw_semaphore termios_rwsem;
struct mutex winsize_mutex;
spinlock_t ctrl_lock;
spinlock_t flow_lock;
/* Termios values are protected by the termios rwsem */
struct ktermios termios, termios_locked;
struct termiox *termiox; /* May be NULL for unsupported */
@ -261,7 +262,7 @@ struct tty_struct {
unsigned long flags;
int count;
struct winsize winsize; /* winsize_mutex */
int stopped;
int stopped; /* flow_lock */
int flow_stopped;
int hw_stopped;
int packet;
@ -400,7 +401,9 @@ extern int tty_paranoia_check(struct tty_struct *tty, struct inode *inode,
extern char *tty_name(struct tty_struct *tty, char *buf);
extern void tty_wait_until_sent(struct tty_struct *tty, long timeout);
extern int tty_check_change(struct tty_struct *tty);
extern void __stop_tty(struct tty_struct *tty);
extern void stop_tty(struct tty_struct *tty);
extern void __start_tty(struct tty_struct *tty);
extern void start_tty(struct tty_struct *tty);
extern int tty_register_driver(struct tty_driver *driver);
extern int tty_unregister_driver(struct tty_driver *driver);

4
include/linux/tty_driver.h
View File

@ -152,6 +152,8 @@
* This routine notifies the tty driver that it should stop
* outputting characters to the tty device.
*
* Called with ->flow_lock held. Serialized with start() method.
*
* Optional:
*
* Note: Call stop_tty not this method.
@ -161,6 +163,8 @@
* This routine notifies the tty driver that it resume sending
* characters to the tty device.
*
* Called with ->flow_lock held. Serialized with stop() method.
*
* Optional:
*
* Note: Call start_tty not this method.