Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752289Ab0KMGZ0 (ORCPT ); Sat, 13 Nov 2010 01:25:26 -0500 Received: from mail-pz0-f46.google.com ([209.85.210.46]:38982 "EHLO mail-pz0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750904Ab0KMGZX (ORCPT ); Sat, 13 Nov 2010 01:25:23 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=date:from:to:cc:subject:message-id:references:mime-version :content-type:content-disposition:content-transfer-encoding :in-reply-to:user-agent; b=C2qzvWdx/vRfJGgpVVUiYzdzN0g/58iQP+Bjin+cmy5x+I1qrMkZx+sKeVHI7EBLxr aUdNbpp8uFzBWlYKSukSCzeGvh1qYrYzQQFj26jFUKp7U/anko+rSSr4b7ZV0x1OMxrN 4HsyVY+Pzv/gDCi2GqoouXQ9UMFPvWa8ixdUk= Date: Sat, 13 Nov 2010 14:28:24 +0800 From: =?utf-8?Q?Am=C3=A9rico?= Wang To: Yong Zhang Cc: =?utf-8?Q?Am=C3=A9rico?= Wang , Eric Dumazet , Cypher Wu , linux-kernel@vger.kernel.org, netdev Subject: Re: Kernel rwlock design, Multicore and IGMP Message-ID: <20101113062824.GC3837@hack> References: <1289489007.17691.1310.camel@edumazet-laptop> <20101112071323.GB5660@cr0.nay.redhat.com> <1289546874.17691.1774.camel@edumazet-laptop> <20101112081945.GA5949@cr0.nay.redhat.com> <20101112091818.GB5949@cr0.nay.redhat.com> <20101112130017.GA9752@zhy> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20101112130017.GA9752@zhy> User-Agent: Mutt/1.5.19 (2009-01-05) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3279 Lines: 81 On Fri, Nov 12, 2010 at 09:00:17PM +0800, Yong Zhang wrote: >On Fri, Nov 12, 2010 at 05:18:18PM +0800, Américo Wang wrote: >> On Fri, Nov 12, 2010 at 05:09:45PM +0800, Yong Zhang wrote: >> >On Fri, Nov 12, 2010 at 4:19 PM, Américo Wang wrote: >> >> On Fri, Nov 12, 2010 at 08:27:54AM +0100, Eric Dumazet wrote: >> >>>Le vendredi 12 novembre 2010 à 15:13 +0800, Américo Wang a écrit : >> >>>> On Fri, Nov 12, 2010 at 11:32:59AM +0800, Cypher Wu wrote: >> >>>> >On Thu, Nov 11, 2010 at 11:23 PM, Eric Dumazet wrote: >> >>>> >> Le jeudi 11 novembre 2010 à 21:49 +0800, Cypher Wu a écrit : >> >>>> >> >> >>>> >> Hi >> >>>> >> >> >>>> >> CC netdev, since you ask questions about network stuff _and_ rwlock >> >>>> >> >> >>>> >> >> >>>> >>> I'm using TILEPro and its rwlock in kernel is a liitle different than >> >>>> >>> other platforms. It have a priority for write lock that when tried it >> >>>> >>> will block the following read lock even if read lock is hold by >> >>>> >>> others. Its code can be read in Linux Kernel 2.6.36 in >> >>>> >>> arch/tile/lib/spinlock_32.c. >> >>>> >> >> >>>> >> This seems a bug to me. >> >>>> >> >> >>>> >> read_lock() can be nested. We used such a schem in the past in iptables >> >>>> >> (it can re-enter itself), >> >>>> >> and we used instead a spinlock(), but with many discussions with lkml >> >>>> >> and Linus himself if I remember well. >> >>>> >> >> >>>> >It seems not a problem that read_lock() can be nested or not since >> >>>> >rwlock doesn't have 'owner', it's just that should we give >> >>>> >write_lock() a priority than read_lock() since if there have a lot >> >>>> >read_lock()s then they'll starve write_lock(). >> >>>> >We should work out a well defined behavior so all the >> >>>> >platform-dependent raw_rwlock has to design under that principle. >> >>>> >> >>> >> >>>AFAIK, Lockdep allows read_lock() to be nested. >> >>> >> >>>> It is a known weakness of rwlock, it is designed like that. :) >> >>>> >> >>> >> >>>Agreed. >> >>> >> >> >> >> Just for record, both Tile and X86 implement rwlock with a write-bias, >> >> this somewhat reduces the write-starvation problem. >> > >> >Are you sure(on x86)? >> > >> >It seems that we never realize writer-bias rwlock. >> > >> >> Try >> >> % grep RW_LOCK_BIAS -nr arch/x86 >> >> *And* read the code to see how it works. :) > >If read_lock()/write_lock() fails, the subtracted value(1 for >read_lock() and RW_LOCK_BIAS for write_lock()) is added back. >So reader and writer will contend on the same lock fairly. > >And RW_LOCK_BIAS based rwlock is a variant of sighed-test >rwlock, so it works in the same way to highest-bit-set mode >rwlock. > >Seem you're cheated by it's name(RW_LOCK_BIAS). :) Ah, no, I made a mistake that I thought the initial value of rwlock is something like 0, but clearly it is RW_LOCK_BIAS. Yeah, then there is certainly no bias to writers, and x86 must be using almost the same algorithm with Tile. -- Live like a child, think like the god. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/