Commit f9e053dc authored by Peter Hurley's avatar Peter Hurley Committed by Greg Kroah-Hartman
Browse files

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: default avatarPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent d7a855bd

drivers/tty/tty_io.c

+28 −11
Original line number Diff line number Diff line
@@ -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);

include/linux/tty.h

+4 −1
Original line number Diff line number Diff line
@@ -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);

include/linux/tty_driver.h

+4 −0
Original line number Diff line number Diff line
@@ -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.

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