tty: Don't take tty_mutex for tty count changes

Holding tty_mutex is no longer required to serialize changes to
the tty_count or to prevent concurrent opens of closing ttys;
tty_lock() is sufficient.
Reviewed-by: NAlan Cox <alan@linux.intel.com>
Signed-off-by: NPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_io.c

@@ -1804,10 +1804,6 @@ int tty_release(struct inode *inode, struct file *filp)
 	 * each iteration we avoid any problems.
 	 */
 	while (1) {
-		/* Guard against races with tty->count changes elsewhere and
-		   opens on /dev/tty */
-
-		mutex_lock(&tty_mutex);
 		tty_lock_pair(tty, o_tty);
 		tty_closing = tty->count <= 1;
 		o_tty_closing = o_tty &&
@@ -1840,7 +1836,6 @@ int tty_release(struct inode *inode, struct file *filp)
 		printk(KERN_WARNING "%s: %s: read/write wait queue active!\n",
 				__func__, tty_name(tty, buf));
 		tty_unlock_pair(tty, o_tty);
-		mutex_unlock(&tty_mutex);
 		schedule();
 	}
 
@@ -1891,7 +1886,6 @@ int tty_release(struct inode *inode, struct file *filp)
 		read_unlock(&tasklist_lock);
 	}
 
-	mutex_unlock(&tty_mutex);
 	tty_unlock_pair(tty, o_tty);
 	/* At this point, the tty->count == 0 should ensure a dead tty
 	   cannot be re-opened by a racing opener */