netdev - Re: Possible deadlock in ipv6?

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Date:	Wed, 6 Jun 2012 20:01:38 +0400
From:	Vladimir Davydov <VDavydov@...allels.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Possible deadlock in ipv6?

On Jun 6, 2012, at 7:58 PM, Eric Dumazet wrote:

> On Wed, 2012-06-06 at 17:53 +0200, Eric Dumazet wrote:
>> On Wed, 2012-06-06 at 18:49 +0400, Vladimir Davydov wrote:
>>> I'm not familiar with the linux net subsystem, so I would appreciate if 
>>> someone could clarify if the following call chain is possible:
>>> 
>>> addrconf_ifdown() calls neigh_ifdown(nd_tbl) which locks nd_tbl.lock for 
>>> writing and calls
>>> 
>>>     pneigh_ifdown
>>>     pndisc_destructor
>>>     ipv6_dev_mc_dec
>>>     __ipv6_dev_mc_dec
>>>     igmp6_group_dropped
>>>     igmp6_leave_group
>>>     igmp6_send
>>>     icmp6_dst_alloc
>>>     ip6_neigh_lookup
>>>     neigh_create
>>> 
>>> and neigh_create() locks nd_tbl.lock for writing again resulting in a 
>>> deadlock.
>> 
>> It seems a deadlock is possible indeed, good catch !
>> 
>> 
> 
> And it seems this neigh_down() can be removed, its called later
> (after dev->ip6_ptr is cleared)
> 

BTW, commit d1ed113f1669390da9898da3beddcc058d938587 did exactly the same, but it was reverted along with a bundle of other commits by 73a8bd74e2618990dbb218c3d82f53e60acd9af0.

> diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
> index 8f6411c..62c4c00 100644
> --- a/net/ipv6/addrconf.c
> +++ b/net/ipv6/addrconf.c
> @@ -2750,7 +2750,6 @@ static int addrconf_ifdown(struct net_device *dev, int how)
> 	ASSERT_RTNL();
> 
> 	rt6_ifdown(net, dev);
> -	neigh_ifdown(&nd_tbl, dev);
> 
> 	idev = __in6_dev_get(dev);
> 	if (idev == NULL)
> 
> 

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