[PATCH, urgent, tty-next] tty: fix __tty_insert_flip_char regression

[Arnd Bergmann]

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@xxxxxxxxx>
Tested-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
---
 drivers/tty/tty_buffer.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

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;
-- 
2.9.0