Date: Fri, 9 Nov 2012 10:59:18 +0100 From: Fabien Thomas <fabien.thomas@netasq.com> To: "Alexander V. Chernikov" <melifaro@FreeBSD.org> Cc: freebsd-net@FreeBSD.org, Andre Oppermann <oppermann@networx.ch>, freebsd-hackers@FreeBSD.org Subject: Re: [patch] reducing arp locking Message-ID: <37E1F76F-D951-4B36-AF00-039DA1CC5CF3@netasq.com> In-Reply-To: <509CC776.9010200@FreeBSD.org> References: <509AEDAC.10002@FreeBSD.org> <509B884F.7040106@networx.ch> <509B88B1.3070905@FreeBSD.org> <49EE4F42-6162-40F4-9DE0-1ACA1289B225@netasq.com> <509CC776.9010200@FreeBSD.org>
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Le 9 nov. 2012 à 10:05, Alexander V. Chernikov a écrit : > On 09.11.2012 12:51, Fabien Thomas wrote: >> >> 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. > I see no problems in copying incorrect mac address in that case: > if host mac address id updated, this is, most likely, another host, and several packets being lost changes nothing. Sending packet to a bogus mac address is not really nothing :) > > However, there can be some realistic scenario where this can be the case (L2 load balancing/failover). I'll update in_arpinput() to do lle removal/insertion in that case. > >> la_preempt modification is also write access to an unlocked structure. > This one changes nothing: > current code does this under _read_ lock. Under the table lock not the entry lock ? Table lock is here to protect the table if I've understood the code correctly. If i get an exclusive reference to the entry you will end up reading and writing to the entry without any lock. > >> >> >>> >>>> 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 >>> >>> >>> _______________________________________________ >>> freebsd-hackers@freebsd.org mailing list >>> http://lists.freebsd.org/mailman/listinfo/freebsd-hackers >>> To unsubscribe, send any mail to "freebsd-hackers-unsubscribe@freebsd.org" >> >> >
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