tty: fix __tty_insert_flip_char regression

Sergey noticed a small but fatal mistake in __tty_insert_flip_char,
leading to an oops in an interrupt handler when using any serial
port.

The problem is that I accidentally took the tty_buffer pointer
before calling __tty_buffer_request_room(), which replaces the
buffer. This moves the pointer lookup to the right place after
allocating the new buffer space.

Fixes: 979990c62848 ("tty: improve tty_insert_flip_char() fast path")
Reported-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Tested-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 6a8563633d4b..f8eba1c5412f 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -372,12 +372,13 @@ EXPORT_SYMBOL(tty_insert_flip_string_flags);
  */
 int __tty_insert_flip_char(struct tty_port *port, unsigned char ch, char flag)
 {
-        struct tty_buffer *tb = port->buf.tail;
+        struct tty_buffer *tb;
         int flags = (flag == TTY_NORMAL) ? TTYB_NORMAL : 0;
 
         if (!__tty_buffer_request_room(port, 1, flags))
                 return 0;
 
+        tb = port->buf.tail;
         if (~tb->flags & TTYB_NORMAL)
                 *flag_buf_ptr(tb, tb->used) = flag;
         *char_buf_ptr(tb, tb->used++) = ch;