netdev - Re: [RFC][PATCH] netconsole: avoid deadlock on printk from driver code

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Date:	Thu, 14 Aug 2008 16:28:37 +0300
From:	"Pekka Enberg" <penberg@...helsinki.fi>
To:	"David Miller" <davem@...emloft.net>
Cc:	vegard.nossum@...il.com, vegardno@....uio.no,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	jgarzik@...ox.com, adobriyan@...il.com
Subject: Re: [RFC][PATCH] netconsole: avoid deadlock on printk from driver code

Hi David,

On Wed, Aug 13, 2008 at 1:37 PM, David Miller <davem@...emloft.net> wrote:
> From: Vegard Nossum <vegardno@....uio.no>
> Date: Wed, 13 Aug 2008 11:53:24 +0200
>
>> [PATCH] netconsole: avoid deadlock on printk from driver code
>>
>> I encountered a hard-to-debug deadlock when I pulled out the plug of my
>> RealTek 8139 which was also running netconsole: The driver wants to print
>> a "link down" message. However, this triggers netconsole, which wants to
>> print the message using the same device. Here is a backtrace:
>
> See my other reply, this is absolutely the wrong way to go about
> this.
>
> You only have two sane options:
>
> 1) Defer the link status printk message into a deferred context,
>   such as a workqueue, so that you can do it outside of the
>   lock.

It's not just the link status. We have assert() and debugging printks
there under tp->lock as well which can also trigger the deadlock
condition.

On Wed, Aug 13, 2008 at 1:37 PM, David Miller <davem@...emloft.net> wrote:
> 2) Do your locking differently so that the link status handling
>   locking does not bisect the locking used for packet transmit
>   in ->hard_start_xmit().
>
> #2 is the reason why most other drivers don't have this silly
> bug, they don't hold TX path locks when handling link status
> and printing out such messages.

Yeah, that works for the link status case, but not for things like the
printks in rtl8139_tx_interrupt()... Hmm.

                             Pekka
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