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Subject: Re: [patch] reducing arp locking
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From: Fabien Thomas <fabien.thomas@netasq.com>
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To: Alexander V. Chernikov <melifaro@FreeBSD.org>
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Le 8 nov. 2012 =E0 11:25, Alexander V. Chernikov a =E9crit :

> On 08.11.2012 14:24, Andre Oppermann wrote:
>> On 08.11.2012 00:24, Alexander V. Chernikov wrote:
>>> Hello list!
>>>=20
>>> Currently we need to acquire 2 read locks to perform simple 6-byte
>>> copying from arp record to packet
>>> ethernet header.
>>>=20
>>> It seems that acquiring lle lock for fast path (main traffic flow) =
is
>>> not necessary even with
>>> current code.
>>>=20
>>> My tests shows ~10% improvement with this patch applied.
>>>=20
>>> If nobody objects I plan to commit this change at the end of next =
week.
>>=20
>> 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.


>=20
>> 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.
>>=20
>=20
>=20
> --=20
> WBR, Alexander
>=20
>=20
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