Subject: Re: [Bugme-new] [Bug 13760] New: 2.6.30 kernel locks up with pppoe in back trace (regression)

From: Pavel Emelyanov <[email-masked]>
Date: Tue, 28 Jul 2009 13:51:54 +0400

Eric Dumazet wrote:
> Igor M Podlesny a écrit :
>> [...]
>>> Could have been a problem in net core, perhaps.
>>>
>>> Below is a ppp fix from 2.6.31, but it seems unlikely to fix your problem.
>>>
>>> It would help if we could see that trace, please.  A digital photo
>>> would suit.
>> 	Here it is:
>>
>> 		http://bugzilla.kernel.org/attachment.cgi?id=22516
>>
>> 	(It's 2.6.30.3)
>> 	
> 
> Looking at this, I believe net_assign_generic() is not safe.
> 
> Two cpus could try to expand/update the array at same time, one update could be lost.
> 
> register_pernet_gen_device() has a mutex to guard against concurrent
> calls, but net_assign_generic() has no locking at all.
> 
> I doubt this is the reason of the crash, still worth to mention it...
> 
> [PATCH] net: net_assign_generic() is not SMP safe
> 
> Two cpus could try to expand/update the array at same time, one update
> could be lost during the copy of old array.

How can this happen? The array is updated only during ->init routines
of the pernet_operations, which are called from under the net_mutex.

Do I miss anything?

> Re-using net_mutex is an easy way to fix this, it was used right
> before to allocate the 'id'
> 
> Signed-off-by: Eric Dumazet <[email-masked]>
> ---
> 
> diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
> index b7292a2..9c31ad1 100644
> --- a/net/core/net_namespace.c
> +++ b/net/core/net_namespace.c
> @@ -467,15 +467,17 @@ int net_assign_generic(struct net *net, int id, void *data)
>  	BUG_ON(!mutex_is_locked(&net_mutex));
>  	BUG_ON(id == 0);
>  
> +	mutex_lock(&net_mutex);
>  	ng = old_ng = net->gen;
>  	if (old_ng->len >= id)
>  		goto assign;
>  
>  	ng = kzalloc(sizeof(struct net_generic) +
>  			id * sizeof(void *), GFP_KERNEL);
> -	if (ng == NULL)
> +	if (ng == NULL) {
> +		mutex_unlock(&net_mutex);
>  		return -ENOMEM;
> -
> +	}
>  	/*
>  	 * Some synchronisation notes:
>  	 *
> @@ -494,6 +496,7 @@ int net_assign_generic(struct net *net, int id, void *data)
>  	call_rcu(&old_ng->rcu, net_generic_release);
>  assign:
>  	ng->ptr[id - 1] = data;
> +	mutex_unlock(&net_mutex);
>  	return 0;
>  }
>  EXPORT_SYMBOL_GPL(net_assign_generic);
> 
>