Messages in this thread | | | Date | Tue, 25 Aug 2009 11:08:57 +0800 | Subject | Re: v2.6.31-rc6: BUG: unable to handle kernel NULL pointer dereference at 0000000000000008 | From | Xiaotian Feng <> |
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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 > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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