From mboxrd@z Thu Jan 1 00:00:00 1970 From: =?utf-8?Q?Am=C3=A9rico?= Wang Subject: Re: Kernel rwlock design, Multicore and IGMP Date: Fri, 12 Nov 2010 15:13:23 +0800 Message-ID: <20101112071323.GB5660@cr0.nay.redhat.com> References: <1289489007.17691.1310.camel@edumazet-laptop> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Eric Dumazet , linux-kernel@vger.kernel.org, netdev To: Cypher Wu Return-path: Received: from mail-vw0-f46.google.com ([209.85.212.46]:45174 "EHLO mail-vw0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750936Ab0KLHIk (ORCPT ); Fri, 12 Nov 2010 02:08:40 -0500 Content-Disposition: inline In-Reply-To: Sender: netdev-owner@vger.kernel.org List-ID: 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 =C3=A0 21:49 +0800, Cypher Wu a =C3=A9crit= : >> >> 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 th= an >>> 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 iptab= les >> (it can re-enter itself), >> and we used instead a spinlock(), but with many discussions with lkm= l >> 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. It is a known weakness of rwlock, it is designed like that. :) The solution is to use RCU or seqlock, but I don't think seqlock is proper for this case you described. So, try RCU lock.