Re: tty_mutex and tty_old_pgrp

[Alan Cox <alan@lxorguk.ukuu.org.uk>]

Ar Maw, 2006-06-27 am 23:29 -0400, ysgrifennodd Jon Smirl:
> Why does this need to be protected? exit.c
> 	mutex_lock(&tty_mutex);
> 	current->signal->tty = NULL;
> 	mutex_unlock(&tty_mutex);

It races against things like a third party haungup of the controlling
tty session if the lock is not held.

> After looking at all of this for a couple of hours it looks to me like
> tty_mutex could be removed if ref counts were used to control when the
> tty_struct gets destroyed. 

You would still want memory barriers and to audit the time things took
effect as there is a fairly defined ordering involved here. Fully
refcounting ttys would not be a bad thing but would require some driver
work because the driver level objects hung off a tty are often not
dynamically allocated and are not themselves refcounted so would get
corrupted if the tty object was freed and a new one allocated and opened
in the meantime.

Alan