netdev - Re: [PATCH] netfilter: use per-cpu spinlock rather than RCU

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <alpine.LFD.2.00.0904131213360.26713@localhost.localdomain>
Date:	Mon, 13 Apr 2009 12:17:00 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Martin Josefsson <gandalf@...fs.se>
cc:	Stephen Hemminger <shemminger@...tta.com>,
	paulmck@...ux.vnet.ibm.com, David Miller <davem@...emloft.net>,
	paulus@...ba.org, mingo@...e.hu, laijs@...fujitsu.com,
	jeff.chua.linux@...il.com, dada1@...mosbay.com, jengelh@...ozas.de,
	kaber@...sh.net, r000n@...0n.net, linux-kernel@...r.kernel.org,
	netfilter-devel@...r.kernel.org, netdev@...r.kernel.org,
	benh@...nel.crashing.org
Subject: Re: [PATCH] netfilter: use per-cpu spinlock rather than RCU

On Mon, 13 Apr 2009, Martin Josefsson wrote:
> 
> Doesn't spin_lock() result in a pipeline flush on x86?

It's about 20-40 cycles when cached and uncontended, with some outliers 
(50+ cycles on P4, 12 cycles on some AMD opterons).

So it's not a big deal if you actually hit that case.

> iirc there was a benchmark in an RCU paper that tested using per cpu
> spin_locks and the result was that it didn't scale well at all.

Spinlocks scale wonderfully well if you only touch them on one CPU.

Of course, if you truly only touch them on one CPU they are pointless, but 
a "all normal code only touches the local CPU spinlock, the really odd 
cases take all locks" approach works fine. It makes the uncommon case 
really quite slow, but if it truly is uncommon, that's fine.

			Linus
--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html