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>

1 files changed, 3 insertions, 7 deletions

diff --git a/drivers/tty/synclink.c b/drivers/tty/synclink.c
index d48e040cd8c5..b7991707ffc0 100644
--- a/drivers/tty/synclink.c
+++ b/drivers/tty/synclink.c
@@ -3267,7 +3267,6 @@ static int block_til_ready(struct tty_struct *tty, struct file * filp,
 	DECLARE_WAITQUEUE(wait, current);
 	int		retval;
 	bool		do_clocal = false;
-	bool		extra_count = false;
 	unsigned long	flags;
 	int		dcd;
 	struct tty_port *port = &info->port;
@@ -3300,10 +3299,7 @@ static int block_til_ready(struct tty_struct *tty, struct file * filp,
 			 __FILE__,__LINE__, tty->driver->name, port->count );
 
 	spin_lock_irqsave(&info->irq_spinlock, flags);
-	if (!tty_hung_up_p(filp)) {
-		extra_count = true;
-		port->count--;
-	}
+	port->count--;
 	spin_unlock_irqrestore(&info->irq_spinlock, flags);
 	port->blocked_open++;
 	
@@ -3342,7 +3338,7 @@ static int block_til_ready(struct tty_struct *tty, struct file * filp,
 	remove_wait_queue(&port->open_wait, &wait);
 	
 	/* FIXME: Racy on hangup during close wait */
-	if (extra_count)
+	if (!tty_hung_up_p(filp))
 		port->count++;
 	port->blocked_open--;
 	
@@ -3403,7 +3399,7 @@ static int mgsl_open(struct tty_struct *tty, struct file * filp)
 			 __FILE__,__LINE__,tty->driver->name, info->port.count);
 
 	/* If port is closing, signal caller to 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));
 		retval = ((info->port.flags & ASYNC_HUP_NOTIFY) ?