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.

This one I agree with (sorry it took a while to get to, I wanted to sit
and think carefully about it as it is subtle and clever)

Can you generate a single patch which reverts the unneeded locks and
adds this instead. Add a Signed-off-by and send it to Greg to get it
into 3.8/3.9 somewhere.

Thanks

Alan