1 files changed, 64 insertions, 0 deletions

diff --git a/drivers/tty/tty_ioctl.c b/drivers/tty/tty_ioctl.c
index d58b92cc187c..132d452578bb 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