Re: tty_mutex and tty_old_pgrp

[Jon Smirl]

On 6/27/06, Paul Fulghum <paulkf@xxxxxxxxxxxxx> wrote:
> No one has leaped in here with any wisdom, but the people
> who wrote that code may be dead or otherwise employed.
>
> If you have knowledge of how those bits work,
> I encourage to you dig through the code and determine
> what needs to be done. It is certainly an area that can
> use more review.
>
> I did see a comment that tty_mutex protects the creation
> and destruction of tty structures, so I assume the coverage
> of tty_old_pgrp has some relation to that. Unfortunately,
> I have seen other locks get borrowed for multiple purposes.

I'm having trouble divining the use of tty_mutex. In most cases it is
protecting pgrp, tty_old_pgrp and tty fields in signal_struct.
sys_setsid(), disassociate_ctty(), daemonize(), tty::release_dev(),
tty_open() use it this way. But it is also used to protect
tty::init_dev() which makes no use of the signal_struct fields. Then
there is the use in vt::con_close() which also does not involved the
signal_struct fields.

Why does this need to be protected? exit.c
	mutex_lock(&tty_mutex);
	current->signal->tty = NULL;
	mutex_unlock(&tty_mutex);

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. I think the problem being addressed is
dangling pointers in signal_struct::tty when a tty is being closed.
Ref counts would delay the freeing of tty_struct until all the
references were released.

--
Jon Smirl
jonsmirl@xxxxxxxxx
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