pty: Narrow the race on ldisc locking

The pty code has always been buggy on its ldisc handling. The recent
changes made the window for the race much bigger. Pending fixing it
properly which is not at all trivial, at least make the race small again so
we don't disrupt other dev work.
Signed-off-by: NAlan Cox <alan@linux.intel.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/char/pty.c

@@ -104,7 +104,7 @@ static int pty_write(struct tty_struct *tty, const unsigned char *buf,
 	struct tty_struct *to = tty->link;
 	int	c;
 
-	if (!to || tty->stopped)
+	if (!to || !to->ldisc || tty->stopped)
 		return 0;
 
 	c = to->receive_room;
@@ -148,7 +148,7 @@ static int pty_chars_in_buffer(struct tty_struct *tty)
 	int count;
 
 	/* We should get the line discipline lock for "tty->link" */
-	if (!to || !to->ldisc->ops->chars_in_buffer)
+	if (!to || !to->ldisc || !to->ldisc->ops->chars_in_buffer)
 		return 0;
 
 	/* The ldisc must report 0 if no characters available to be read */
@@ -183,7 +183,7 @@ static void pty_flush_buffer(struct tty_struct *tty)
 	struct tty_struct *to = tty->link;
 	unsigned long flags;
 
-	if (!to)
+	if (!to || !to->ldisc)
 		return;
 
 	if (to->ldisc->ops->flush_buffer)