linux-kernel - Re: [PATCHv5 05/17] tty: don't allow ldisc dcd

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Date:	Thu, 25 Nov 2010 14:03:54 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Alexander Gordeev <lasaine@....cs.msu.su>
Cc:	Alan Cox <alan@...ux.intel.com>, linux-kernel@...r.kernel.org,
	"Nikita V\. Youshchenko" <yoush@...msu.su>,
	linuxpps@...enneenne.com, Rodolfo Giometti <giometti@...eenne.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Arnd Bergmann <arnd@...db.de>,
	Al Viro <viro@...iv.linux.org.uk>,
	Nick Piggin <npiggin@...nel.dk>,
	Jason Wessel <jason.wessel@...driver.com>,
	Philippe Langlais <philippe.langlais@...ricsson.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCHv5 05/17] tty: don't allow ldisc dcd_change() after ldisc
 halt

> 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

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