Re: 2.6.17-rc4-mm3-lockdep BUG: possible deadlock detected!

[Andrew Morton <akpm@osdl.org>]

On Mon, 29 May 2006 20:32:47 +0200
"Michal Piotrowski" <michal.k.k.piotrowski@gmail.com> wrote:

> I get this with Ingo's lockdep patch from
> http://people.redhat.com/mingo/generic-irq-subsystem/
> 
> ====================================
> [ BUG: possible deadlock detected! ]
> ------------------------------------
> modprobe/592 is trying to acquire lock:
>  (&grp->list_mutex){----}, at: [<fd9ee555>]
> snd_seq_port_connect+0xc0/0x337 [snd_seq]
> 
> but task is already holding lock:
>  (&grp->list_mutex){----}, at: [<fd9ee4fb>]
> snd_seq_port_connect+0x66/0x337 [snd_seq]
> 
> which could potentially lead to deadlocks!
> 
> other info that might help us debug this:
> 1 locks held by modprobe/592:
>  #0:  (&grp->list_mutex){----}, at: [<fd9ee4fb>]
> snd_seq_port_connect+0x66/0x337 [snd_seq]

yes,

        down_write(&src->list_mutex);
        down_write(&dest->list_mutex);

I wonder if there's anything which prevents another task from concurrently
coming in and trying to perform the opposite connection and causing a
deadlock.

The way we normally handle this is

	if (src < dest) {
		down_write(&src->list_mutex);
		down_write(&dest->list_mutex);
	} else {
		down_write(&dest->list_mutex);
		down_write(&src->list_mutex);
	}