tty: Replace ldisc locking with ldisc_sem

Line discipline locking was performed with a combination of
a mutex, a status bit, a count, and a waitqueue -- basically,
a rw semaphore.

Replace the existing combination with an ld_semaphore.

Fixes:
 1) the 'reference acquire after ldisc locked' bug
 2) the over-complicated halt mechanism
 3) lock order wrt. tty_lock()
 4) dropping locks while changing ldisc
 5) previously unidentified deadlock while locking ldisc from
    both linked ttys concurrently
 6) previously unidentified recursive deadlocks

Adds much-needed lockdep diagnostics.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 3 insertions, 4 deletions

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index fa80af3656a3..0fa5db4c7a7a 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1388,8 +1388,7 @@ static int tty_reopen(struct tty_struct *tty)
         struct tty_driver *driver = tty->driver;
 
         if (test_bit(TTY_CLOSING, &tty->flags) ||
-                        test_bit(TTY_HUPPING, &tty->flags) ||
-                        test_bit(TTY_LDISC_CHANGING, &tty->flags))
+                        test_bit(TTY_HUPPING, &tty->flags))
                 return -EIO;
 
         if (driver->type == TTY_DRIVER_TYPE_PTY &&
@@ -1405,7 +1404,7 @@ static int tty_reopen(struct tty_struct *tty)
         }
         tty->count++;
 
-        WARN_ON(!test_bit(TTY_LDISC, &tty->flags));
+        WARN_ON(!tty->ldisc);
 
         return 0;
 }
@@ -3017,7 +3016,7 @@ void initialize_tty_struct(struct tty_struct *tty,
         tty->pgrp = NULL;
         mutex_init(&tty->legacy_mutex);
         mutex_init(&tty->termios_mutex);
-        mutex_init(&tty->ldisc_mutex);
+        init_ldsem(&tty->ldisc_sem);
         init_waitqueue_head(&tty->write_wait);
         init_waitqueue_head(&tty->read_wait);
         INIT_WORK(&tty->hangup_work, do_tty_hangup);