Subject: Re: [PATCH][XFRM] Replace rwlock on xfrm_policy_afinfo with rcu

From: Fan Du <[email-masked]>
Date: Wed, 8 Aug 2012 14:44:02 +0800

First, sorry to jump in.

On 2012年08月08日 14:25, Eric Dumazet wrote:
> On Tue, 2012-08-07 at 10:51 +0530, Priyanka Jain wrote:
>> xfrm_policy_afinfo is read mosly data structure.
>> Write on xfrm_policy_afinfo is done only at the
>> time of configuration.
>> So rwlocks can be safely replaced with RCU.
>>
>> RCUs usage optimizes the performance.
>
>
>>   static struct xfrm_policy_afinfo *xfrm_policy_get_afinfo(unsigned short family)
>> @@ -2530,16 +2535,16 @@ static struct xfrm_policy_afinfo *xfrm_policy_get_afinfo(unsigned short family)
>>   	struct xfrm_policy_afinfo *afinfo;
>>   	if (unlikely(family>= NPROTO))
>>   		return NULL;
>> -	read_lock(&xfrm_policy_afinfo_lock);
>> -	afinfo = xfrm_policy_afinfo[family];
>> +	rcu_read_lock();
>> +	afinfo = rcu_dereference(xfrm_policy_afinfo[family]);
>>   	if (unlikely(!afinfo))
>> -		read_unlock(&xfrm_policy_afinfo_lock);
>> +		rcu_read_unlock();
>
> This makes no sense to me : We cant safely return afinfo here.
>
> Note the current code is buggy as well, this is worrying.
>
> As soon as we exit from xfrm_policy_get_afinfo(), pointer might be
> invalid.
>
Yes, it might be invalid, but all the callers have checked the return
value, thus use it in a sane way.
So I don't follow "Note the current code is buggy as well".

Am I missing something here?

> Really, RCU conversion should be the right moment to spot those bugs and
> first fix them (for stable trees)
>
>
>
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-- 

Love each day!
--fan