netdev - Re: [PATCH (regression)] Fragments: fix race between inet_frag_find and inet_frag_secret

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Date:	Wed, 25 Jun 2008 10:42:56 +0400
From:	Pavel Emelyanov <xemul@...nvz.org>
To:	Jarek Poplawski <jarkao2@...il.com>
CC:	David Miller <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: [PATCH (regression)] Fragments: fix race between	inet_frag_find
 and   inet_frag_secret_rebuild

Jarek Poplawski wrote:
> Pavel Emelyanov wrote, On 06/24/2008 12:43 PM:
> 
>> The problem is that while we work w/o the inet_frags.lock even
>> read-locked the secret rebuild timer may occur (on another CPU,
> - since BHs are still disables in the inet_frag_find) and change 
> 
> + since BHs are still disabled in the inet_frag_find) and change 
> 
>> the rnd seed for ipv4/6 fragments.
>>
>> It was caused by my patch fd9e63544cac30a34c951f0ec958038f0529e244
>> ([INET]: Omit double hash calculations in xxx_frag_intern) late 
>> in the 2.6.24 kernel, so this should probably be queued to -stable.
>>
>> Signed-off-by: Pavel Emelyanov <xemul@...nvz.org>
>>
>> ---
>>
>> diff --git a/net/ipv4/inet_fragment.c b/net/ipv4/inet_fragment.c
>> index 4ed429b..0546a0b 100644
>> --- a/net/ipv4/inet_fragment.c
>> +++ b/net/ipv4/inet_fragment.c
>> @@ -192,14 +192,21 @@ EXPORT_SYMBOL(inet_frag_evictor);
>>  
>>  static struct inet_frag_queue *inet_frag_intern(struct netns_frags *nf,
>>  		struct inet_frag_queue *qp_in, struct inet_frags *f,
>> -		unsigned int hash, void *arg)
>> +		void *arg)
>>  {
>>  	struct inet_frag_queue *qp;
>>  #ifdef CONFIG_SMP
>>  	struct hlist_node *n;
>>  #endif
>> +	unsigned int hash;
>>  
>>  	write_lock(&f->lock);
>> +	/*
>> +	 * While we stayed w/o the lock other CPU could update
>> +	 * the rnd seed, so we need to re-calculate the hash
>> +	 * chain. Fortunatelly the qp_in can be used to get one.
>> +	 */
>> +	hash = f->hashfn(qp_in);
>>  #ifdef CONFIG_SMP
>>  	/* With SMP race we have to recheck hash table, because
>>  	 * such entry could be created on other cpu, while we
> 
> Maybe it's a matter of taste: since there is this "#ifdef CONFIG_SMP",
> and the new comment concerns with "other CPU", why this re-calculation
> isn't done only for SMP? 

Because the hash value is required also *outside* this ifdef and adding
a fancier logic is probably not good for a -rc7 fix.

However, I will re-consider this for net-next.

> And, btw., probably __acquires/__releases annotations could be added
> with this patch.

This is also a net-next material (I hope Dave agrees with me on both).

> Regards,
> Jarek P.
> 

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