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

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Date:	Tue, 25 Aug 2009 11:08:57 +0800
From:	Xiaotian Feng <xtfeng@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org, x86@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Greg Kroah-Hartman <gregkh@...e.de>
Subject: Re: v2.6.31-rc6: BUG: unable to handle kernel NULL pointer 
	dereference at 0000000000000008

On Tue, Aug 25, 2009 at 8:09 AM, Linus
Torvalds<torvalds@...ux-foundation.org> 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.

I have run the test case for about 2 hours on my x86_64 machine, no
panic happens.

>
>                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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