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

Alan Cox
2009-10-13T23:11:31Z

> > 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.
>
> Hmm. Those tty's have too many different locks already.
>
> But maybe we could just have one generic mutex, and use it for termios and
> IO locking. It makes perfect sense to serialize the ->receive_buf() code
> with any termios changes, since termios is what affects _how_ that
> ->receive_buf() function works.

You cannot trivially just take the same lock for receive_buf and termios
locking at the moment. The reason is that receive_buf can cause the tty to
throttle which causes us to call the throttle methods which take the lock.

tty_throttle() and tty_unthrottle() can be called from both receive_buf
and non receive_buf paths so you can't just remove it.

The better existing lock is probably tty->ldisc_mutex which we take when
doing ldisc changes (which are an even more dramatic change during
receive_buf). We don't do ldisc changes from the receive_buf path and it
opens a path for further simplification of the ldisc logic if we can get
to the point where the ldisc doesn't get called randomly from the tty
layer when changing.

> I do wonder why tiocsti() doesn't just use the tty buffering layer,
> though? Maybe that harks back to the whole "pty's did things differently"
> thing? Why does it go directly to ->receive_buf() in the first place?

Historical question - I don't know - and at the time I commented it there
was no quick fix and bigger problems to sort first

Alan