Re: [Bug #14388] keyboard under X with 2.6.31

Alan Cox
2009-10-13T21:32:12Z

> But flush_to_ldisc() itself has a real oddity: it uses "tty->buf.lock" to
> protect everything, BUT NOT THE ACTUAL CALL TO ->receive_buf()!

Indeed or it deadlocks

> Anyway, the above explanation "feels right". It would easily explain the
> behavior, because if the ->receive_buf() calls get re-ordered, then the
> events get re-ordered, and one simple case of that would be to see the key
> "release" event before the key "press" event.

And you would only see it in X11 because only X11 deals in raw key events.

> The sane fix would be to just run ->receive_buf() under the tty->buf.lock,
> but I assume we'd have a lot of unhappy ldiscs if we did that (and
> possibly irq latency problems too).

You bet

However there is nothing stopping you stuffing that lot into a per tty
mutex solely used for serializing those submissions. It can't really be a
mutex for anything else as we call back into the ldisc to send stuff. You
aren't allowed to stuff data into the ldisc unless it can sleep so a
mutex is fine.

I can't help feeling a mutex might be simpler. It would also then fix
tiocsti() which is most definitely broken right now and documented as
racing.

Alan