check_usage_backwards() && forwards? (Was: [2.6.33-rc5] starting emacs makes lockdep warning)

[Oleg Nesterov <oleg@redhat.com>]

(add  lockdep gurus)

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

On 01/26, KOSAKI Motohiro wrote:
>
> =========================================================
> [ 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).

Typo in check_usage_backwards() ?

Oleg.

--- a/kernel/lockdep.c
+++ b/kernel/lockdep.c
@@ -2147,7 +2147,7 @@ check_usage_backwards(struct task_struct
 		return ret;
 
 	return print_irq_inversion_bug(curr, &root, target_entry,
-					this, 1, irqclass);
+					this, 0, irqclass);
 }
 
 void print_irqtrace_events(struct task_struct *curr)