From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754239AbZHYCtT (ORCPT ); Mon, 24 Aug 2009 22:49:19 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754136AbZHYCtT (ORCPT ); Mon, 24 Aug 2009 22:49:19 -0400 Received: from rv-out-0506.google.com ([209.85.198.237]:47405 "EHLO rv-out-0506.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754034AbZHYCtS (ORCPT ); Mon, 24 Aug 2009 22:49:18 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=date:from:to:cc:subject:message-id:references:mime-version :content-type:content-disposition:in-reply-to:user-agent; b=nVtpgZ2rX/ZBu1jtvm1F40Lt1AZn7yH5U0MPxExyumId9MfQt6m3eEclacVXTIczOD K9MEiIEOgfSjGq0yoUBzIzEgO4MDc8EQk+zzMkcy8Z2wqaKDxa210vF5VbQcr1DqXBZd Rx8uufDbkxNhLmXftmI2fCcVuwAtIAEI5AT34= Date: Tue, 25 Aug 2009 10:48:49 +0800 From: Dave Young To: Linus Torvalds Cc: "Eric W. Biederman" , linux-kernel@vger.kernel.org, x86@kernel.org, Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Alan Cox , Greg Kroah-Hartman Subject: Re: v2.6.31-rc6: BUG: unable to handle kernel NULL pointer dereference at 0000000000000008 Message-ID: <20090825024849.GA2156@darkstar> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.19 (2009-01-05) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > -- > 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/