tty: Fix missed wakeup from packet mode status update

The pty master read() can miss the wake up for a packet mode
status change. For example,

CPU 0                                   | CPU 1
n_tty_read()                            | n_tty_packet_mode_flush()
  ...                                   |   .
  if (packet & link->ctrl_status) {     |   .
    /* no new ctrl_status ATM */        |   .
                                        |   spin_lock
                                        |     ctrl_status |= TIOCPKT_FLUSHREAD
                                        |   spin_unlock
                                        |   wake_up(link->read_wait)
  }                                     |
  set_current_state(TASK_INTERRUPTIBLE) |
  ...                                   |

The pty master read() will now sleep (assuming there is no input) having
missed the read_wait wakeup.

Set the task state before the condition test.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Reviewed-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index cd725cc6e21e0b340a79d5780ec9de31c5ed6b39..7f134b977c36173cdb31b4281dd5cb0e3c2c552f 100644 (file)
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -2168,6 +2168,11 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
 
        add_wait_queue(&tty->read_wait, &wait);
        while (nr) {
+               /* This statement must be first before checking for input
+                  so that any interrupt will set the state back to
+                  TASK_RUNNING. */
+               set_current_state(TASK_INTERRUPTIBLE);
+
                /* First test for status change. */
                if (packet && tty->link->ctrl_status) {
                        unsigned char cs;
@@ -2185,10 +2190,6 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
                        nr--;
                        break;
                }
-               /* This statement must be first before checking for input
-                  so that any interrupt will set the state back to
-                  TASK_RUNNING. */
-               set_current_state(TASK_INTERRUPTIBLE);
 
                if (((minimum - (b - buf)) < ldata->minimum_to_wake) &&
                    ((minimum - (b - buf)) >= 1))