> It seems that tty was already free in disassocate_ctty when it tries
> to dereference tty->driver.
>
> After moving the lock_kernel before the mutex_unlock, I can no longer
> reproduce the problem.
>
> Please review and consider to apply:

This doesn't help as the BKL doesn't protect tty here - we don't have
refcounting on the ttys yet so this bug (which goes back forever) simply
becomes a different sized race if you swap the lock ordering.

Given tty_vhangup just fires off a wait queue which is killed on the tty
destruction you should be able for now at least to move the mutex_unlock
to after the call to tty_vhangup().

Does that also fix the problem ?

Alan
--
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/

Am Donnerstag, 7. August 2008 schrieb Alan Cox:
> > It seems that tty was already free in disassocate_ctty when it tries
> > to dereference tty->driver.
> >
> > After moving the lock_kernel before the mutex_unlock, I can no longer
> > reproduce the problem.
> >
> > Please review and consider to apply:
>
> This doesn't help as the BKL doesn't protect tty here - we don't have
> refcounting on the ttys yet so this bug (which goes back forever) simply
> becomes a different sized race if you swap the lock ordering.
>
> Given tty_vhangup just fires off a wait queue which is killed on the tty
> destruction you should be able for now at least to move the mutex_unlock
> to after the call to tty_vhangup().
>
> Does that also fix the problem ?

You mean something like the below patch? Yes, that works as well.

Signed-off-by: Christian Borntraeger

---
drivers/char/tty_io.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

Index: kvm/drivers/char/tty_io.c
================================================== =================
--- kvm.orig/drivers/char/tty_io.c
+++ kvm/drivers/char/tty_io.c
@@ -1161,12 +1161,11 @@ void disassociate_ctty(int on_exit)
tty = get_current_tty();
if (tty) {
tty_pgrp = get_pid(tty->pgrp);
- mutex_unlock(&tty_mutex);
lock_kernel();
- /* XXX: here we race, there is nothing protecting tty */
if (on_exit && tty->driver->type != TTY_DRIVER_TYPE_PTY)
tty_vhangup(tty);
unlock_kernel();
+ mutex_unlock(&tty_mutex);
} else if (on_exit) {
struct pid *old_pgrp;
spin_lock_irq(&current->sighand->siglock);
--
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/

> Please review and consider to apply:
>
> Signed-off-by: Christian Borntraeger
> ---
> drivers/char/tty_io.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> Index: kvm/drivers/char/tty_io.c
> ================================================== =================
> --- kvm.orig/drivers/char/tty_io.c
> +++ kvm/drivers/char/tty_io.c
> @@ -1161,8 +1161,8 @@ void disassociate_ctty(int on_exit)
> tty = get_current_tty();
> if (tty) {
> tty_pgrp = get_pid(tty->pgrp);
> - mutex_unlock(&tty_mutex);
> lock_kernel();
> + mutex_unlock(&tty_mutex);
> /* XXX: here we race, there is nothing protecting tty */
> if (on_exit && tty->driver->type != TTY_DRIVER_TYPE_PTY)
> tty_vhangup(tty);

For upstream (see -next tree) I have pushed a refcounting solution (or at
least I hope its a solution) to the ttydev tree. For 2.6.26 your approach
partially fixes this and certainly doesn't make it worse so I'll now push
it onwards.

Alan
--
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/