tty: Remove tty_hung_up_p() tests from tty drivers' open()

Since at least before 2.6.30, it has not been possible to observe
a hung up file pointer in a tty driver's open() method unless/until
the driver open() releases the tty_lock() (eg., before blocking).

This is because tty_open() adds the file pointer while holding
the tty_lock() _and_ doesn't release the lock until after calling
the tty driver's open() method. [ Before tty_lock(), this was
lock_kernel(). ]

Since __tty_hangup() first waits on the tty_lock() before
enumerating and hanging up the open file pointers, either
__tty_hangup() will wait for the tty_lock() or tty_open() will
not yet have added the file pointer. For example,

CPU 0                          |  CPU 1
                               |
tty_open                       |  __tty_hangup
  ..                           |    ..
  tty_lock                     |    ..
  tty_reopen                   |    tty_lock  / blocks
  ..                           |
  tty_add_file(tty, filp)      |
  ..                           |
  tty->ops->open(tty, filp)    |
    tty_port_open              |
      tty_port_block_til_ready |
        ..                     |
        while (1)              |
          ..                   |
          tty_unlock           |    / unblocks
          schedule             |    for each filp on tty->tty_files
                               |      f_ops = tty_hung_up_fops;
                               |    ..
                               |    tty_unlock
          tty_lock             |
  ..                           |
  tty_unlock                   |

Note that since tty_port_block_til_ready() and similar drop
the tty_lock while blocking, when woken, the file pointer
must then be tested for having been hung up.

Also, fix bit-rotted drivers that used extra_count to track the
port->count bump.

CC: Mikael Starvik <starvik@axis.com>
CC: Samuel Ortiz <samuel@sortiz.org>
CC: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Acked-by: Jesper Nilsson <jesper.nilsson@axis.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

2 files changed, 5 insertions, 18 deletions

diff --git a/drivers/tty/serial/crisv10.c b/drivers/tty/serial/crisv10.c
index d567ac5d3af4..58e6f61a87e4 100644
--- a/drivers/tty/serial/crisv10.c
+++ b/drivers/tty/serial/crisv10.c
@@ -3831,14 +3831,13 @@ block_til_ready(struct tty_struct *tty, struct file * filp,
 	DECLARE_WAITQUEUE(wait, current);
 	unsigned long	flags;
 	int		retval;
-	int		do_clocal = 0, extra_count = 0;
+	int		do_clocal = 0;
 
 	/*
 	 * If the device is in the middle of being closed, then block
 	 * until it's done, and then try again.
 	 */
-	if (tty_hung_up_p(filp) ||
-	    (info->port.flags & ASYNC_CLOSING)) {
+	if (info->port.flags & ASYNC_CLOSING) {
 		wait_event_interruptible_tty(tty, info->port.close_wait,
 			!(info->port.flags & ASYNC_CLOSING));
 #ifdef SERIAL_DO_RESTART
@@ -3879,10 +3878,7 @@ block_til_ready(struct tty_struct *tty, struct file * filp,
 	       info->line, info->port.count);
 #endif
 	local_irq_save(flags);
-	if (!tty_hung_up_p(filp)) {
-		extra_count++;
-		info->port.count--;
-	}
+	info->port.count--;
 	local_irq_restore(flags);
 	info->port.blocked_open++;
 	while (1) {
@@ -3921,7 +3917,7 @@ block_til_ready(struct tty_struct *tty, struct file * filp,
 	}
 	set_current_state(TASK_RUNNING);
 	remove_wait_queue(&info->port.open_wait, &wait);
-	if (extra_count)
+	if (!tty_hung_up_p(filp))
 		info->port.count++;
 	info->port.blocked_open--;
 #ifdef SERIAL_DEBUG_OPEN
@@ -3976,8 +3972,7 @@ rs_open(struct tty_struct *tty, struct file * filp)
 	/*
 	 * If the port is in the middle of closing, bail out now
 	 */
-	if (tty_hung_up_p(filp) ||
-	    (info->port.flags & ASYNC_CLOSING)) {
+	if (info->port.flags & ASYNC_CLOSING) {
 		wait_event_interruptible_tty(tty, info->port.close_wait,
 			!(info->port.flags & ASYNC_CLOSING));
 #ifdef SERIAL_DO_RESTART
diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 3e08df52d68d..1a5fbf7ab347 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -1584,14 +1584,6 @@ static int uart_open(struct tty_struct *tty, struct file *filp)
 	tty_port_tty_set(port, tty);
 
 	/*
-	 * If the port is in the middle of closing, bail out now.
-	 */
-	if (tty_hung_up_p(filp)) {
-		retval = -EAGAIN;
-		goto err_dec_count;
-	}
-
-	/*
 	 * Start up the serial port.
 	 */
 	retval = uart_startup(tty, state, 0);