2 files changed, 82 insertions, 0 deletions

diff --git a/drivers/tty/tty_ioctl.c b/drivers/tty/tty_ioctl.c
index d58b92cc187..132d452578b 100644
--- a/drivers/tty/tty_ioctl.c
+++ b/drivers/tty/tty_ioctl.c
@@ -106,6 +106,7 @@ void tty_throttle(struct tty_struct *tty)
 	if (!test_and_set_bit(TTY_THROTTLED, &tty->flags) &&
 	    tty->ops->throttle)
 		tty->ops->throttle(tty);
+	tty->flow_change = 0;
 	mutex_unlock(&tty->termios_mutex);
 }
 EXPORT_SYMBOL(tty_throttle);
@@ -129,11 +130,74 @@ void tty_unthrottle(struct tty_struct *tty)
 	if (test_and_clear_bit(TTY_THROTTLED, &tty->flags) &&
 	    tty->ops->unthrottle)
 		tty->ops->unthrottle(tty);
+	tty->flow_change = 0;
 	mutex_unlock(&tty->termios_mutex);
 }
 EXPORT_SYMBOL(tty_unthrottle);
 
 /**
+ *	tty_throttle_safe	-	flow control
+ *	@tty: terminal
+ *
+ *	Similar to tty_throttle() but will only attempt throttle
+ *	if tty->flow_change is TTY_THROTTLE_SAFE. Prevents an accidental
+ *	throttle due to race conditions when throttling is conditional
+ *	on factors evaluated prior to throttling.
+ *
+ *	Returns 0 if tty is throttled (or was already throttled)
+ */
+
+int tty_throttle_safe(struct tty_struct *tty)
+{
+	int ret = 0;
+
+	mutex_lock(&tty->termios_mutex);
+	if (!test_bit(TTY_THROTTLED, &tty->flags)) {
+		if (tty->flow_change != TTY_THROTTLE_SAFE)
+			ret = 1;
+		else {
+			__set_bit(TTY_THROTTLED, &tty->flags);
+			if (tty->ops->throttle)
+				tty->ops->throttle(tty);
+		}
+	}
+	mutex_unlock(&tty->termios_mutex);
+
+	return ret;
+}
+
+/**
+ *	tty_unthrottle_safe	-	flow control
+ *	@tty: terminal
+ *
+ *	Similar to tty_unthrottle() but will only attempt unthrottle
+ *	if tty->flow_change is TTY_UNTHROTTLE_SAFE. Prevents an accidental
+ *	unthrottle due to race conditions when unthrottling is conditional
+ *	on factors evaluated prior to unthrottling.
+ *
+ *	Returns 0 if tty is unthrottled (or was already unthrottled)
+ */
+
+int tty_unthrottle_safe(struct tty_struct *tty)
+{
+	int ret = 0;
+
+	mutex_lock(&tty->termios_mutex);
+	if (test_bit(TTY_THROTTLED, &tty->flags)) {
+		if (tty->flow_change != TTY_UNTHROTTLE_SAFE)
+			ret = 1;
+		else {
+			__clear_bit(TTY_THROTTLED, &tty->flags);
+			if (tty->ops->unthrottle)
+				tty->ops->unthrottle(tty);
+		}
+	}
+	mutex_unlock(&tty->termios_mutex);
+
+	return ret;
+}
+
+/**
  *	tty_wait_until_sent	-	wait for I/O to finish
  *	@tty: tty we are waiting for
  *	@timeout: how long we will wait
diff --git a/include/linux/tty.h b/include/linux/tty.h
index c75d886b030..189ca80494d 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -258,6 +258,7 @@ struct tty_struct {
 	unsigned char warned:1;
 	unsigned char ctrl_status;	/* ctrl_lock */
 	unsigned int receive_room;	/* Bytes free for queue */
+	int flow_change;
 
 	struct tty_struct *link;
 	struct fasync_struct *fasync;
@@ -318,6 +319,21 @@ struct tty_file_private {
 
 #define TTY_WRITE_FLUSH(tty) tty_write_flush((tty))
 
+/* Values for tty->flow_change */
+#define TTY_THROTTLE_SAFE 1
+#define TTY_UNTHROTTLE_SAFE 2
+
+static inline void __tty_set_flow_change(struct tty_struct *tty, int val)
+{
+	tty->flow_change = val;
+}
+
+static inline void tty_set_flow_change(struct tty_struct *tty, int val)
+{
+	tty->flow_change = val;
+	smp_mb();
+}
+
 #ifdef CONFIG_TTY
 extern void console_init(void);
 extern void tty_kref_put(struct tty_struct *tty);
@@ -400,6 +416,8 @@ extern int tty_write_room(struct tty_struct *tty);
 extern void tty_driver_flush_buffer(struct tty_struct *tty);
 extern void tty_throttle(struct tty_struct *tty);
 extern void tty_unthrottle(struct tty_struct *tty);
+extern int tty_throttle_safe(struct tty_struct *tty);
+extern int tty_unthrottle_safe(struct tty_struct *tty);
 extern int tty_do_resize(struct tty_struct *tty, struct winsize *ws);
 extern void tty_driver_remove_tty(struct tty_driver *driver,
 				  struct tty_struct *tty);