Re: [PATCHv5 05/17] tty: don't allow ldisc dcd_change() after ldisc halt

Alan Cox
2010-11-25T14:07:22Z

> Didn't know that current design is crap. :)

It's based on the old big kernel lock and it has lots of horrors when
used from interrupt paths as a result. A big chunk of it can in time I
think move to just having a per tty ldisc lock that covers all changes
etc, but we aren't there yet.

> Ok, I think I'll add a new waitqueue and a new bit (TTY_LDISC_NOREF)

We've got the wait queue already

My first guess would be something like

static struct tty_ldisc *tty_ldisc_irqget(struct tty_struct *tty)
{
unsigned long flags;
struct tty_ldisc *ld;

spin_lock_irqsave(&tty_ldisc_lock, flags);
ld = NULL;
if (test_bit(TTY_LDISC, &tty->flags)) {
ld = get_ldisc(tty->ldisc);
WARN_ON(test_and_set_bit(TTY_LDISC_IRQ, &tty->flags));
}
spin_unlock_irqrestore(&tty_ldisc_lock, flags);
return ld;
}

static struct tty_ldisc *tty_ldisc_irqput(struct tty_ldisc *ld)
{
clear_bit(TTY_LDISC_IRQ, &tty->flags);
tty_ldisc_put(ld);
}

(and I think you can then wait on tty_ldisc_idle for the flag to go clear
after clearing TTY_LDISC)

It might be better to make it a count of course in case there are later
multiple users of this ?

Alan