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

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Date:	Tue, 28 Jul 2009 14:30:33 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Pavel Emelyanov <xemul@...nvz.org>
CC:	Igor M Podlesny <for.poige+bugzilla.kernel.org@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	bugzilla-daemon@...zilla.kernel.org,
	bugme-daemon@...zilla.kernel.org, netdev@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [Bugme-new] [Bug 13760] New: 2.6.30 kernel locks up with pppoe
 in 	back trace (regression)

Pavel Emelyanov a écrit :
> 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?
> 

Oops, I missed the obvious "BUG_ON(!mutex_is_locked(&net_mutex));"

Sorry for the noise and untested patch as well :)

>> 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 <eric.dumazet@...il.com>
>> ---
>>
>> 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);
--
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