netdev - Re: [PATCH][PPPOL2TP]: Fix SMP oops in pppol2tp driver

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Date:	Tue, 19 Feb 2008 09:03:12 +0000
From:	James Chapman <jchapman@...alix.com>
To:	David Miller <davem@...emloft.net>
CC:	jarkao2@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH][PPPOL2TP]: Fix SMP oops in pppol2tp driver

David Miller wrote:
> From: James Chapman <jchapman@...alix.com>
> Date: Mon, 18 Feb 2008 22:09:24 +0000
> 
>> Here's a new version of the patch. The patch avoids disabling irqs
>> and fixes the sk_dst_get() usage that DaveM mentioned. But even with
>> this patch, lockdep still complains if hundreds of ppp sessions are
>> inserted into a tunnel as rapidly as possible (lockdep trace is
>> below). I can stop these errors by wrapping the call to ppp_input()
>> in pppol2tp_recv_dequeue_skb() with local_irq_save/restore. What is
>> a better fix?
> 
> Firstly, let's fix one thing at a time.  Leave the sk_dst_get()
> thing alone until we can prove that it's part of the lockdep
> traces.

In reproducing the problem, I obtained several lockdep traces that 
implicated sk_dst_get(). I changed the code to use __sk_dst_check() as 
you suggested and they went away. At that point, I was hopeful the 
locking issues were fixed. But after several minutes of 
creating/deleting hundreds of ppp sessions, lockdep dumped another 
error. It is that error that I posted yesterday.

> Next, I can't see why ppp_input() needs to be invoked with
> interrupts disabled.  There are many other things that invoke
> that in software interrupt context, such as pppoe.

I agree. I'm seeking advice on what the underlying cause is of this new 
trace.

> Please provide the lockdep traces without the ppp_input() IRQ
> disabling so this can be properly analyzed.

The trace _was_ without ppp_input IRQ disabling. The trace doesn't occur 
if I disable IRQs around the ppp_input() call. The patch I sent showed 
the changes I made before running the tests that created the new lockdep 
trace. I'm sorry this wasn't clear.

-- 
James Chapman
Katalix Systems Ltd
http://www.katalix.com
Catalysts for your Embedded Linux software development

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