Re: [BUG] 2.6.38-rc2: Circular Locking Dependency

Paul Mackerras
2011-02-07T10:29:59Z

On Sun, Feb 06, 2011 at 11:28:56PM -0800, David Miller wrote:
> From: Knut Petersen <Knut_Petersen@t-online.de>
> Date: Mon, 24 Jan 2011 10:25:55 +0100
>
> > As I was hunting something different I found the following (potential)
> > problem on an openSuSE 11.3 system with kernel 2.6.38-rc2.
> > The message is triggerd by smpppd starting a dsl connection.
> >
> > Knut
> >
> >
> > NET: Registered protocol family 24
> >
> > =======================================================
> > [ INFO: possible circular locking dependency detected ]
> > 2.6.38-rc2-kape #7
> > -------------------------------------------------------
> > pppd/2529 is trying to acquire lock:
> > (&(&pch->downl)->rlock){+.....}, at: [<f814a634>] ppp_push+0x59/0x4a8
> > [ppp_generic]
> >
> > but task is already holding lock:
> > (&(&ppp->wlock)->rlock){+.-...}, at: [<f814ae1b>]
> > ppp_xmit_process+0x19/0x451 [ppp_generic]
> >
> > which lock already depends on the new lock.
>
> I've stared over this trace several times and can't figure out what
> the problem is.
>
> Paul, any idea?

We seem to have recursed in the ppp code because of (apparently)
handling a softirq inside a spin_lock_bh region. :( If I understand
the original report correctly, the stack trace looks like this in part:

[<c04153eb>] net_rx_action+0x3f/0xfe
[<c0128563>] __do_softirq+0x76/0xfd
-> #1 (_xmit_NETROM){+.-...}:
[<c01462b2>] lock_acquire+0x47/0x5e
[<c0471c9c>] _raw_spin_lock_irqsave+0x2e/0x3e
[<c040ed60>] skb_dequeue+0x12/0x4a
[<f814c237>] ppp_channel_push+0x2e/0x94 [ppp_generic]

So we were in ppp_channel_push, and the first thing it does is
spin_lock_bh(&pch->downl), and then it calls skb_dequeue, which did a
spin_lock_irqsave, and then somehow we get into __do_softirq. I
thought spin_lock_bh should have stopped softirqs from running?

Paul.