Re: v2.6.31-rc6: BUG: unable to handle kernel NULL pointer dereference at 0000000000000008

[Dave Young <hidave.darkstar@gmail.com>]

On Mon, Aug 24, 2009 at 05:09:08PM -0700, Linus Torvalds wrote:
> 
> 
> On Mon, 24 Aug 2009, Linus Torvalds wrote:
> > 
> > But I wanted to let people know that the patch is clearly not the "last 
> > word" on this. It's a useful thing to try, but we need something better.
> 
> This may be better (this is a replacement for the previous patch).
> 
> Instead of using 'cancel_delayed_work_sync()', it makes tty_ldisc_hangup() 
> do a 'flush_scheduled_work()' afterwards, like the other callers already 
> do.
> 
> And like 'tty_ldisc_release()' already does, it does this all before even 
> getting the ldisc_mutex, avoiding the deadlock.
> 
> I'm not 100% happy with this patch either, but my remaining unhappiness is 
> more with the tty locking in general that causes this all. I suspect this 
> patch in itself is not any worse than the other hacks we have.
> 
> Oh, and in case you didn't guess - this is _STILL_ totally untested. It 
> compiles for me, but that's all I'm going to guarantee. I'm just looking 
> at the code (and getting pretty fed up with it ;)
> 
> And as already mentioned: I doubt the deadlock on tty->ldisc_mutex is 
> anything that would be hit in practice. And even if it can be triggered, 
> the previous patch I sent out is still interesting in a "does it make the 
> problem go away" sense. Because if it doesn't (with or without a new 
> deadlock), then I'm looking at all the wrong places.

Tested for half an hour, seems it fixed the problem.

> 
> 		Linus
> 
> ---
>  drivers/char/tty_ldisc.c |   10 +++++++---
>  1 files changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/char/tty_ldisc.c b/drivers/char/tty_ldisc.c
> index 1733d34..f893d18 100644
> --- a/drivers/char/tty_ldisc.c
> +++ b/drivers/char/tty_ldisc.c
> @@ -508,8 +508,9 @@ static void tty_ldisc_restore(struct tty_struct *tty, struct tty_ldisc *old)
>   *	be obtained while the delayed work queue halt ensures that no more
>   *	data is fed to the ldisc.
>   *
> - *	In order to wait for any existing references to complete see
> - *	tty_ldisc_wait_idle.
> + *	You need to do a 'flush_scheduled_work()' (outside the ldisc_mutex
> + *	in order to make sure any currently executing ldisc work is also
> + *	flushed.
>   */
>  
>  static int tty_ldisc_halt(struct tty_struct *tty)
> @@ -753,11 +754,14 @@ void tty_ldisc_hangup(struct tty_struct *tty)
>  	 * N_TTY.
>  	 */
>  	if (tty->driver->flags & TTY_DRIVER_RESET_TERMIOS) {
> +		/* Make sure the old ldisc is quiescent */
> +		tty_ldisc_halt(tty);
> +		flush_scheduled_work();
> +
>  		/* Avoid racing set_ldisc or tty_ldisc_release */
>  		mutex_lock(&tty->ldisc_mutex);
>  		if (tty->ldisc) {	/* Not yet closed */
>  			/* Switch back to N_TTY */
> -			tty_ldisc_halt(tty);
>  			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
> --
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