Lockdep has found the real bug, but the output doesn't look right to me:

> =========================================================
> [ INFO: possible irq lock inversion dependency detected ]
> 2.6.33-rc5 #77
> ---------------------------------------------------------
> emacs/1609 just changed the state of lock:
>  (&(&tty->ctrl_lock)->rlock){+.....}, at: [<ffffffff8127c648>] tty_fasync+0xe8/0x190
> but this lock took another, HARDIRQ-unsafe lock in the past:
>  (&(&sighand->siglock)->rlock){-.....}

"HARDIRQ-unsafe" and "this lock took another" looks wrong, afaics.

>   ... key      at: [<ffffffff81c054a4>] __key.46539+0x0/0x8
>   ... acquired at:
>    [<ffffffff81089af6>] __lock_acquire+0x1056/0x15a0
>    [<ffffffff8108a0df>] lock_acquire+0x9f/0x120
>    [<ffffffff81423012>] _raw_spin_lock_irqsave+0x52/0x90
>    [<ffffffff8127c1be>] __proc_set_tty+0x3e/0x150
>    [<ffffffff8127e01d>] tty_open+0x51d/0x5e0

The stack-trace shows that this lock (ctrl_lock) was taken under
->siglock (which is hopefully irq-safe).

This is a clear typo in check_usage_backwards() where we tell the print a
fancy routine we're forwards.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20100126181641.GA10460@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>