netdev - Re: possible circular locking dependency in ISDN PPP

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Date:	Wed, 21 Oct 2009 18:24:42 +0200
From:	Tilman Schmidt <tilman@...p.cc>
To:	Xiaotian Feng <xtfeng@...il.com>
CC:	LKML <linux-kernel@...r.kernel.org>,
	isdn4linux <isdn4linux@...tserv.isdn4linux.de>,
	Netdev <netdev@...r.kernel.org>,
	Karsten Keil <isdn@...ux-pingi.de>
Subject: Re: possible circular locking dependency in ISDN PPP

Thanks for your analysis. The patch you propose does indeed prevent the
"circular locking dependency" message, with no noticeable ill effect.

I cannot say why xmit_lock was taken around isdn_net_lp_busy() in the
first place. The ISDN4Linux people would be the ones to comment on that.
If none of them objects, I propose you add a Signed-off-by line to your
patch and submit it to Karsten Keil, the ISDN maintainer, for inclusion.
You can also add a "Tested-by: Tilman Schmidt <tilman@...p.cc>" line.

Thanks,
Tilman

Am 19.10.2009 07:27 schrieb Xiaotian Feng:
> So there's a circular locking dependency.. Looking into isdn_net_get_locked_lp()
[...]
> Why do we need to hold xmit_lock while using
> isdn_net_lp_busy(nd->queue) ? Can following patch fix this bug?
> 
> ---
> diff --git a/drivers/isdn/i4l/isdn_net.h b/drivers/isdn/i4l/isdn_net.h
> index 74032d0..7511f08 100644
> --- a/drivers/isdn/i4l/isdn_net.h
> +++ b/drivers/isdn/i4l/isdn_net.h
> @@ -83,19 +83,19 @@ static __inline__ isdn_net_local *
> isdn_net_get_locked_lp(isdn_net_dev *nd)
> 
>         spin_lock_irqsave(&nd->queue_lock, flags);
>         lp = nd->queue;         /* get lp on top of queue */
> -       spin_lock(&nd->queue->xmit_lock);
>         while (isdn_net_lp_busy(nd->queue)) {
> -               spin_unlock(&nd->queue->xmit_lock);
>                 nd->queue = nd->queue->next;
>                 if (nd->queue == lp) { /* not found -- should never happen */
>                         lp = NULL;
>                         goto errout;
>                 }
> -               spin_lock(&nd->queue->xmit_lock);
>         }
>         lp = nd->queue;
>         nd->queue = nd->queue->next;
> +       spin_unlock_irqrestore(&nd->queue_lock, flags);
> +       spin_lock(&lp->xmit_lock);
>         local_bh_disable();
> +       return lp;
>  errout:
>         spin_unlock_irqrestore(&nd->queue_lock, flags);
>         return lp;
> 

-- 
Tilman Schmidt                    E-Mail: tilman@...p.cc
Bonn, Germany
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