tty: Add safe tty throttle/unthrottle functions

The tty driver can become stuck throttled due to race conditions
between throttle and unthrottle, when the decision to throttle
or unthrottle is conditional. The following example helps to
illustrate the race:

  CPU 0                        |  CPU 1
                               |
if (condition A)               |
                               | <processing such that A not true>
                               | if (!condition A)
                               |     unthrottle()
    throttle()                 |
                               |

Note the converse is also possible; ie.,

  CPU 0                        |  CPU 1
                               |
                               | if (!condition A)
<processing such that A true>  |
if (condition A)               |
    throttle()                 |
                               |     unthrottle()
                               |

Add new throttle/unthrottle functions based on the familiar model
of task state and schedule/wake. For example,

    while (1) {
        tty_set_flow_change(tty, TTY_THROTTLE_SAFE);
        if (!condition)
            break;
        if (!tty_throttle_safe(tty))
            break;
    }
    __tty_set_flow_change(tty, 0);

In this example, if an unthrottle occurs after the condition is
evaluated but before tty_throttle_safe(), then tty_throttle_safe()
will return non-zero, looping and forcing the re-evaluation of
condition.

Reported-by: Vincent Pillet <vincentx.pillet@intel.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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