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tty: Fix missed wakeup from packet mode status update

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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>
This commit is contained in:
Peter Hurley 2014-10-16 15:33:29 -04:00 committed by Greg Kroah-Hartman
parent 4ed60fc257
commit 1aa1bf1115
1 changed files with 5 additions and 4 deletions
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9
drivers/tty/n_tty.c
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@ -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))