netdev - Re: [PATCH v2 1/1] ppp: Fix one deadlock issue of PPP when send frame

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Date:	Wed, 17 Aug 2016 19:42:19 +0200
From:	Guillaume Nault <g.nault@...halink.fr>
To:	fgao@...ai8.com
Cc:	paulus@...ba.org, linux-ppp@...r.kernel.org,
	netdev@...r.kernel.org, gfree.wind@...il.com
Subject: Re: [PATCH v2 1/1] ppp: Fix one deadlock issue of PPP when send frame

On Tue, Aug 16, 2016 at 07:33:38PM +0800, fgao@...ai8.com wrote:
> From: Gao Feng <fgao@...ai8.com>
> 
> PPP channel holds one spinlock before send frame. But the skb may
> select the same PPP channel with wrong route policy. As a result,
> the skb reaches the same channel path. It tries to get the same
> spinlock which is held before. Bang, the deadlock comes out.
> 
Unless I misunderstood the problem you're trying to solve, this patch
doesn't really help: deadlock still occurs if the same IP is used for
L2TP and PPP's peer address.

> Now add one lock owner to avoid it like xmit_lock_owner of
> netdev_queue. Check the lock owner before try to get the spinlock.
> If the current cpu is already the owner, needn't lock again. When
> PPP channel holds the spinlock at the first time, it sets owner
> with current CPU ID.
>
I think you should forbid lock recursion entirely, and drop the packet
if the owner tries to re-acquire the channel lock. Otherwise you just
move the deadlock down the stack (l2tp_xmit_skb() can't be called
recursively).

> diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
> index 70cfa06..6909ab1 100644
> --- a/drivers/net/ppp/ppp_generic.c
> +++ b/drivers/net/ppp/ppp_generic.c
> @@ -162,6 +162,37 @@ struct ppp {
>  			 |SC_MULTILINK|SC_MP_SHORTSEQ|SC_MP_XSHORTSEQ \
>  			 |SC_COMP_TCP|SC_REJ_COMP_TCP|SC_MUST_COMP)
>  
> +struct chennel_lock {
            ^
I guess you meant 'channel_lock'.

> +	spinlock_t lock;
> +	u32 owner;
This field's default value is -1. So why not declaring it as 'int'?