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

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Message-ID: <20091014001131.302d3272@lxorguk.ukuu.org.uk>
Date:	Wed, 14 Oct 2009 00:11:31 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Boyan <btanastasov@...oo.co.uk>,
	"Frédéric L. W. Meunier" <fredlwm@...il.com>,
	"Justin P. Mattock" <justinmattock@...il.com>,
	Nix <nix@...eri.org.uk>, Paul Fulghum <paulkf@...rogate.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kernel Testers List <kernel-testers@...r.kernel.org>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Ed Tomlinson <edt@....ca>,
	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Subject: Re: [Bug #14388] keyboard under X with 2.6.31

> > 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
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