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SubjectRe: 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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