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