tty: Change tty lock order to master->slave

When releasing the master pty, the slave pty also needs to be locked
to prevent concurrent tty count changes for the slave pty and to
ensure that only one parallel master and slave release observe the
final close, and proceed to destruct the pty pair. Conversely, when
releasing the slave pty, locking the master pty is not necessary
(since the master's state can be inferred by the slave tty count).

Introduce tty_lock_slave()/tty_unlock_slave() which acquires/releases
the tty lock of the slave pty. Remove tty_lock_pair()/tty_unlock_pair().

Dropping the tty_lock is no longer required to re-establish a stable
lock order.

Reviewed-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 13 insertions, 19 deletions

diff --git a/drivers/tty/tty_mutex.c b/drivers/tty/tty_mutex.c
index 2e41abebbcba..f43e995c7a0f 100644
--- a/drivers/tty/tty_mutex.c
+++ b/drivers/tty/tty_mutex.c
@@ -4,6 +4,11 @@
 #include <linux/semaphore.h>
 #include <linux/sched.h>
 
+/*
+ * Nested tty locks are necessary for releasing pty pairs.
+ * The stable lock order is master pty first, then slave pty.
+ */
+
 /* Legacy tty mutex glue */
 
 enum {
@@ -45,29 +50,18 @@ void __lockfunc tty_unlock(struct tty_struct *tty)
 }
 EXPORT_SYMBOL(tty_unlock);
 
-/*
- * Getting the big tty mutex for a pair of ttys with lock ordering
- * On a non pty/tty pair tty2 can be NULL which is just fine.
- */
-void __lockfunc tty_lock_pair(struct tty_struct *tty,
-					struct tty_struct *tty2)
+void __lockfunc tty_lock_slave(struct tty_struct *tty)
 {
-	if (tty < tty2) {
-		tty_lock(tty);
-		tty_lock_nested(tty2, TTY_MUTEX_NESTED);
-	} else {
-		if (tty2 && tty2 != tty)
-			tty_lock(tty2);
+	if (tty && tty != tty->link) {
+		WARN_ON(!mutex_is_locked(&tty->link->legacy_mutex) ||
+			!tty->driver->type == TTY_DRIVER_TYPE_PTY ||
+			!tty->driver->type == PTY_TYPE_SLAVE);
 		tty_lock_nested(tty, TTY_MUTEX_NESTED);
 	}
 }
-EXPORT_SYMBOL(tty_lock_pair);
 
-void __lockfunc tty_unlock_pair(struct tty_struct *tty,
-						struct tty_struct *tty2)
+void __lockfunc tty_unlock_slave(struct tty_struct *tty)
 {
-	tty_unlock(tty);
-	if (tty2 && tty2 != tty)
-		tty_unlock(tty2);
+	if (tty && tty != tty->link)
+		tty_unlock(tty);
 }
-EXPORT_SYMBOL(tty_unlock_pair);