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Date: Mon, 7 Feb 2011 21:43:15 +1100 From: Paul Mackerras <paulus@...ba.org> To: David Miller <davem@...emloft.net> Cc: Knut_Petersen@...nline.de, linux-kernel@...r.kernel.org, mostrows@...thlink.net, linux-ppp@...r.kernel.org Subject: Re: [BUG] 2.6.38-rc2: Circular Locking Dependency On Mon, Feb 07, 2011 at 09:29:50PM +1100, Paul Mackerras wrote: > 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? OK, I think I have misinterpreted the lockdep info in the original message. If it's saying that we are trying to get ppp->rlock when we have taken chan->downl, then that would indeed be a bug, since the lock ordering as documented in the comments is ppp->rlock -> chan->downl. I can't see in the code where that happens though and the lockdep trace doesn't seem to be telling me either. Paul. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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