linux-kernel - Re: WARNING at: drivers/char/tty

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Date:	Mon, 3 Aug 2009 01:15:56 +0300
From:	Sergey Senozhatsky <sergey.senozhatsky@...l.by>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Greg KH <greg@...ah.com>
Subject: Re: WARNING at: drivers/char/tty_ldisc.c

On (08/02/09 13:20), Linus Torvalds wrote:
> On Sun, 2 Aug 2009, Sergey Senozhatsky wrote:
> > 
> > non-SMP system 'fails' as well.
> 
> Ahh, can you trigger this reliably? Is it 100% of the time when you shut 
> down from single user mode? Or just occasionally?
> 
Yes, 100% in both SMP and non-SMP builds.

> I take it that you can't get a trace with sysrq-T because nothing gets 
> logged, 
TRUE

> and you don't have a serial port console? 
I don't have a serial port console.


I think there is no need in 
	mutex_unlock(&tty->ldisc_mutex); // HACK
	...
	mutex_lock(&tty->ldisc_mutex); // HACK

since we have tty_ldisc.c:794
	mutex_lock(&tty->ldisc_mutex);			<<
        if (tty->ldisc) { /* Not yet closed */

Anyway, code looks like this:
//JUST FOR TEST!

if (tty->driver->flags & TTY_DRIVER_RESET_TERMIOS) {
		/* Avoid racing set_ldisc or tty_ldisc_release */
		//mutex_lock(&tty->ldisc_mutex);		// Commented
		if (tty->ldisc) {	/* Not yet closed */
			int work = 0;
			/* Switch back to N_TTY */
			mutex_unlock(&tty->ldisc_mutex);        // HACK

			work = tty_ldisc_halt(tty);		//to be sure
			if (work) {
				WARN_ON(0 != 1);
				wait_event_timeout(tty_ldisc_wait,
					   (1 == 0), 15 * HZ);	//Just to read what is written on the screen
			}
			
			if (tty_ldisc_wait_idle(tty) != 0) {
				WARN_ON(0 != 1);
				wait_event_timeout(tty_ldisc_wait,
					   (1 == 0), 15 * HZ);	//Just to read what is written on the screen
			}
			mutex_lock(&tty->ldisc_mutex);          // HACK
			tty_ldisc_reinit(tty);
			/* At this point we have a closed ldisc and we want to
			   reopen it. We could defer this to the next open but
			   it means auditing a lot of other paths so this is
			   a FIXME */
			WARN_ON(tty_ldisc_open(tty, tty->ldisc));
			tty_ldisc_enable(tty);
		}
		mutex_unlock(&tty->ldisc_mutex);
		tty_reset_termios(tty);
	}

This code gave me WARNINGS:
tty_io.c:1267		//tty_reopen: WARN_ON(!test_bit(TTY_LDISC, &tty->flags));
tty_ldisc.c:808		//tty_ldisc_wait_idle
tty_ldisc.c:209		//tty_ldisc_put


	Sergey

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