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    SubjectRe: v2.6.31-rc6: BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
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    On Tue, Aug 25, 2009 at 8:09 AM, Linus
    Torvalds<torvalds@linux-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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