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>

2 files changed, 2 insertions, 5 deletions

diff --git a/include/linux/tty.h b/include/linux/tty.h
index 01ac30efd6a6..7269daf7632b 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -238,7 +238,7 @@ struct tty_struct {
         int index;
 
         /* Protects ldisc changes: Lock tty not pty */
-        struct mutex ldisc_mutex;
+        struct ld_semaphore ldisc_sem;
         struct tty_ldisc *ldisc;
 
         struct mutex atomic_write_lock;
@@ -305,8 +305,6 @@ struct tty_file_private {
 #define TTY_DO_WRITE_WAKEUP     5       /* Call write_wakeup after queuing new */
 #define TTY_PUSH                6       /* n_tty private */
 #define TTY_CLOSING             7       /* ->close() in progress */
-#define TTY_LDISC               9       /* Line discipline attached */
-#define TTY_LDISC_CHANGING      10      /* Line discipline changing */
 #define TTY_LDISC_OPEN          11      /* Line discipline is open */
 #define TTY_PTY_LOCK            16      /* pty private */
 #define TTY_NO_WRITE_SPLIT      17      /* Preserve write boundaries to driver */
diff --git a/include/linux/tty_ldisc.h b/include/linux/tty_ldisc.h
index a1b048999821..23bdd9debb84 100644
--- a/include/linux/tty_ldisc.h
+++ b/include/linux/tty_ldisc.h
@@ -203,8 +203,7 @@ struct tty_ldisc_ops {
 
 struct tty_ldisc {
         struct tty_ldisc_ops *ops;
-        atomic_t users;
-        wait_queue_head_t wq_idle;
+        struct tty_struct *tty;
 };
 
 #define TTY_LDISC_MAGIC 0x5403