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

From: Jarek Poplawski <[email-masked]>
Date: Wed, 13 Feb 2008 07:29:08 +0000

On Tue, Feb 12, 2008 at 10:00:03PM -0800, David Miller wrote:
> From: James Chapman <[email-masked]>
> Date: Tue, 12 Feb 2008 10:58:21 +0000
> 
> > Here is a trace from when we had _bh locks.
> 
> The problem is that the pppol2tp code calls sk_dst_get() in software
> interrupt context and that is not allowed.

Actually, this lockdep report probably warns of something else:
sk_dst_lock which is seen in process context with softirqs enabled
implicitly endangers some other (ppp_generic) locks, which depend
on ppp_generic pch->upl read_lock, which is taken in softirq context.

I can't see on this report any hardirqs dependancies, so it seems even
blocking hard interrupts, just like in this patch, shouldn't solve
this problem: if BHs were really disabled in exactly the same places,
then it seems this warning should trigger in both variants.

I studied long ago some similar problem with pppoe, and it looked like
ppp_generic needed some fix, but now I've to recall or re-learn almost
all and it needs more time... Anyway, there is a possibility, this
warning isn't so dangerous.

> sk_dst_get() grabs sk->sk_dst_lock without any BH enabling or
> disabling.
> 
> It can do that because the usage is to make all the lock
> taking calls in user context, and in the packet processing
> paths use __sk_dst_get().

BTW, does it mean that ip4_datagram_connect() can be used only in user
context?

> Probably what the pppol2tp code should do is use __sk_dst_check()
> instead of sk_dst_get().  You then have to be able to handle
> NULL returns, just like UDP sendmsg() does, which means you'll
> need to cook up a routing lookup if __sk_dst_check() gives you
> NULL because the route became obsolete.

I think that without changing ppp_generic or changing ip_queue_xmit()
to something else in pppol2tp this would be really hard to avoid such
a warning (and maybe not very necessary).

Regards,
Jarek P.