netdev - RE: [PATCH][XFRM] Replace rwlock on xfrm_policy

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Date:	Fri, 10 Aug 2012 04:28:04 +0000
From:	Jain Priyanka-B32167 <B32167@...escale.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Fan Du <fan.du@...driver.com>
Subject: RE: [PATCH][XFRM] Replace rwlock on xfrm_policy_afinfo with rcu

Hello Eric Dumazet,

Like Fan Du, even I could not see the bug in the current code. Can you please elaborate.

Thanks
Priyanka

-----Original Message-----
From: Fan Du [mailto:fan.du@...driver.com] 
Sent: Wednesday, August 08, 2012 12:14 PM
To: Eric Dumazet
Cc: Jain Priyanka-B32167; netdev@...r.kernel.org
Subject: Re: [PATCH][XFRM] Replace rwlock on xfrm_policy_afinfo with rcu

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