Subject: Re: [PATCH 2/2] udp: RCU handling for Unicast packets.

From: Eric Dumazet <[email-masked]>
Date: Wed, 29 Oct 2008 17:09:53 +0100

Corey Minyard a écrit :
> Eric Dumazet wrote:
>> Corey Minyard found a race added in commit 
>> 271b72c7fa82c2c7a795bc16896149933110672d
>> (udp: RCU handling for Unicast packets.)
>>
>> "If the socket is moved from one list to another list in-between the 
>> time  the hash is calculated and the next field is accessed, and the 
>> socket  has moved to the end of the new list, the traversal will not 
>> complete  properly on the list it should have, since the socket will 
>> be on the end  of the new list and there's not a way to tell it's on a 
>> new list and  restart the list traversal.  I think that this can be 
>> solved by  pre-fetching the "next" field (with proper barriers) before 
>> checking the  hash."
>>
>> This patch corrects this problem, introducing a new 
>> sk_for_each_rcu_safenext()
>> macro.
> You also need the appropriate smp_wmb() in udp_lib_get_port() after 
> sk_hash is set, I think, so the next field is guaranteed to be changed 
> after the hash value is changed.
> 
> 

Not sure about this one Corey.

If a reader catches previous value of item->sk_hash, two cases are to be taken into :

1) its udp_hashfn(net, sk->sk_hash) is != hash  
  -> goto begin : Reader will redo its scan

2) its udp_hashfn(net, sk->sk_hash) is == hash
  -> next pointer is good enough : it points to next item in same hash chain.
     No need to rescan the chain at this point.
     Yes we could miss the fact that a new port was bound and this UDP message could be lost.

If we force a smp_wmb(), reader would fetch pointer to begin of list.

1) its udp_hashfn(net, sk->sk_hash) is != hash  
  -> goto begin : Reader will redo its scan : next pointer value had no meaning

2) its udp_hashfn(net, sk->sk_hash) is == hash
  ->next pointer "force reader" to rescan the chain, but wont find new items.

In any case, we cannot lost an UDP message sent to a stable port (previously bound)

Thanks