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

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Date:	Mon, 24 Aug 2009 17:09:08 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
cc:	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 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.

		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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