Commit 1aa1bf11 authored Oct 16, 2014 by Peter Hurley Committed by Greg Kroah-Hartman Nov 05, 2014

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>

parent 4ed60fc2

drivers/tty/n_tty.c

+5 −4

Original line number	Diff line number	Diff line
		@@ -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))

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