[patch] reducing arp locking

[Fabien Thomas]

Le 8 nov. 2012 à 11:25, Alexander V. Chernikov a écrit :

> On 08.11.2012 14:24, Andre Oppermann wrote:
>> On 08.11.2012 00:24, Alexander V. Chernikov wrote:
>>> Hello list!
>>> 
>>> Currently we need to acquire 2 read locks to perform simple 6-byte
>>> copying from arp record to packet
>>> ethernet header.
>>> 
>>> It seems that acquiring lle lock for fast path (main traffic flow) is
>>> not necessary even with
>>> current code.
>>> 
>>> My tests shows ~10% improvement with this patch applied.
>>> 
>>> If nobody objects I plan to commit this change at the end of next week.
>> 
>> This is risky and prone to race conditions.  The copy of the MAC address
>> should be done while the table read lock is held to protect against the
> It is done exactly as you say: table read lock is held.

How do you protect from entry update if i've a ref to the entry ?
You can end up doing bcopy of a partial mac address.
la_preempt modification is also write access to an unlocked structure.


> 
>> entry going away.  You can either return with table lock held and drop
>> it after the copy, or you could a modified lookup function that takes a
>> pointer for the copy destination, do the copy with the read lock, and then
>> return.  If no entry is found an error is returned and obviously no copy
>> is done.
>> 
> 
> 
> -- 
> WBR, Alexander
> 
> 
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