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

From: James Chapman <[email-masked]>
Date: Wed, 20 Feb 2008 16:02:52 +0000

Jarek Poplawski wrote:
> On Mon, Feb 18, 2008 at 10:09:24PM +0000, James Chapman wrote:
> ...
>> Unfortunately the ISP's syslog stops. But I've been able to borrow
>> two Quad Xeon boxes and have reproduced the problem.
>>
>> 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?
> 
> I send here my proposal: it's intended for testing and to check one of
> possible solutions here. IMHO your lockdep reports show there is no
> use to change anything around sk_dst_lock: it would need the global
> change of this lock to fix this problem. So the fix should be done
> around pch->upl lock and this means changing ppp_generic.

Hmm, I need to study the lockdep report again. It seems I'm misreading 
the lockdep output. :(

> In the patch below I've used trylock in places which seem to allow
> for skipping some things (while config is changed only) or simply
> don't need this lock because there is no ppp struct. This could be
> modified to add some waiting loop if necessary. Another option is to
> change the write side of this lock: it looks like more vulnerable if
> something missed because there are more locks involved, but probably
> should be enough to solve this problem too.
> 
> I think pppol2tp need to be first checked only with hlist_lock bh
> patch, unless there were some lockdep reports on these other locks
> too. (BTW, I added ppp maintainer to CC - I hope we get Paul's opinion
> on this.)

I tried your ppp_generic patch with only the hlist_lock bh patch in 
pppol2tp and it seems to fix the ppp create/delete issue. However, when 
I added much more traffic into the test (flood pings over ppp interfaces 
while repeatedly creating/deleting the L2TP (PPP) sessions) I get a soft 
lockup detected in pppol2tp_xmit() after anything between 1 minute and 
an hour. :( I'm investigating that now.

Thanks for your help!

> (testing patch #1)
> ---
> 
>  drivers/net/ppp_generic.c |   33 +++++++++++++++++++++++----------
>  1 files changed, 23 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/net/ppp_generic.c b/drivers/net/ppp_generic.c
> index 4dc5b4b..5cbc534 100644
> --- a/drivers/net/ppp_generic.c
> +++ b/drivers/net/ppp_generic.c
> @@ -1473,7 +1473,7 @@ void
>  ppp_input(struct ppp_channel *chan, struct sk_buff *skb)
>  {
>  	struct channel *pch = chan->ppp;
> -	int proto;
> +	int proto, locked;
>  
>  	if (!pch || skb->len == 0) {
>  		kfree_skb(skb);
> @@ -1481,8 +1481,13 @@ ppp_input(struct ppp_channel *chan, struct sk_buff *skb)
>  	}
>  
>  	proto = PPP_PROTO(skb);
> -	read_lock_bh(&pch->upl);
> -	if (!pch->ppp || proto >= 0xc000 || proto == PPP_CCPFRAG) {
> +	/*
> +	 * We use trylock to avoid dependency between soft-irq-safe upl lock
> +	 * and soft-irq-unsafe sk_dst_lock.
> +	 */
> +	local_bh_disable();
> +	locked = read_trylock(&pch->upl);
> +	if (!locked || !pch->ppp || proto >= 0xc000 || proto == PPP_CCPFRAG) {
>  		/* put it on the channel queue */
>  		skb_queue_tail(&pch->file.rq, skb);
>  		/* drop old frames if queue too long */
> @@ -1493,7 +1498,10 @@ ppp_input(struct ppp_channel *chan, struct sk_buff *skb)
>  	} else {
>  		ppp_do_recv(pch->ppp, skb, pch);
>  	}
> -	read_unlock_bh(&pch->upl);
> +
> +	if (locked)
> +		read_unlock(&pch->upl);
> +	local_bh_enable();
>  }
>  
>  /* Put a 0-length skb in the receive queue as an error indication */
> @@ -1506,16 +1514,18 @@ ppp_input_error(struct ppp_channel *chan, int code)
>  	if (!pch)
>  		return;
>  
> -	read_lock_bh(&pch->upl);
> -	if (pch->ppp) {
> +	/* a trylock comment in ppp_input() */
> +	local_bh_disable();
> +	if (read_trylock(&pch->upl) && pch->ppp) {
>  		skb = alloc_skb(0, GFP_ATOMIC);
>  		if (skb) {
>  			skb->len = 0;		/* probably unnecessary */
>  			skb->cb[0] = code;
>  			ppp_do_recv(pch->ppp, skb, pch);
>  		}
> +		read_unlock(&pch->upl);
>  	}
> -	read_unlock_bh(&pch->upl);
> +	local_bh_enable();
>  }
>  
>  /*
> @@ -2044,10 +2054,13 @@ int ppp_unit_number(struct ppp_channel *chan)
>  	int unit = -1;
>  
>  	if (pch) {
> -		read_lock_bh(&pch->upl);
> -		if (pch->ppp)
> +		/* a trylock comment in ppp_input() */
> +		local_bh_disable();
> +		if (read_trylock(&pch->upl) && pch->ppp) {
>  			unit = pch->ppp->file.index;
> -		read_unlock_bh(&pch->upl);
> +			read_unlock(&pch->upl);
> +		}
> +		local_bh_enable();
>  	}
>  	return unit;
>  }
> --

-- 
James Chapman
Katalix Systems Ltd
http://www.katalix.com
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