Sorry, In you reply not all patch.
Main idea here -  we never flash last (struct tty_buffer) in the
active buffer. Only data for ldisc. (tty->buf.head->read =
tty->buf.head->commit). At that moment driver can collect(write) data
in buffer without conflict.

Ah.. now I understand. Yes that makes sense.  I will think about that
carefully. This is why a good explanation with a patch is important.