[PATCH -next 2/2] tty: Correct tty buffer flush.

[Ilya Zykov <ilya@ilyx.ru>]

  The root of problem is carelessly zeroing pointer(in function __tty_buffer_flush()),
when another thread can use it. It can be cause of "NULL pointer dereference".
  Main idea of the patch, this is never free last (struct tty_buffer) in the active buffer.
Only flush the data for ldisc(tty->buf.head->read = tty->buf.head->commit).
At that moment driver can collect(write) data in buffer without conflict.
It is repeat behavior of flush_to_ldisc(), only without feeding data to ldisc.

Signed-off-by: Ilya Zykov <ilya@ilyx.ru>
---
diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 7602df8..8a3333d 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -119,11 +119,14 @@ static void __tty_buffer_flush(struct tty_port *port)
 	struct tty_bufhead *buf = &port->buf;
 	struct tty_buffer *thead;
 
-	while ((thead = buf->head) != NULL) {
-		buf->head = thead->next;
-		tty_buffer_free(port, thead);
+	if (unlikely(buf->head == NULL))
+		return;
+	while ((thead = buf->head->next) != NULL) {
+		tty_buffer_free(port, buf->head);
+		buf->head = thead;
 	}
-	buf->tail = NULL;
+	WARN_ON(buf->head != buf->tail);
+	buf->head->read = buf->head->commit;
 }
 
 /**