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 17:18:18 +0800 Message-ID: <20101112091818.GB5949@cr0.nay.redhat.com> 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> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: =?utf-8?Q?Am=C3=A9rico?= Wang , Eric Dumazet , Cypher Wu , linux-kernel@vger.kernel.org, netdev To: Yong Zhang Return-path: Received: from mail-qy0-f174.google.com ([209.85.216.174]:44021 "EHLO mail-qy0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752444Ab0KLJNi (ORCPT ); Fri, 12 Nov 2010 04:13:38 -0500 Content-Disposition: inline In-Reply-To: Sender: netdev-owner@vger.kernel.org List-ID: On Fri, Nov 12, 2010 at 05:09:45PM +0800, Yong Zhang wrote: >On Fri, Nov 12, 2010 at 4:19 PM, Am=C3=A9rico Wang wrote: >> On Fri, Nov 12, 2010 at 08:27:54AM +0100, Eric Dumazet wrote: >>>Le vendredi 12 novembre 2010 =C3=A0 15:13 +0800, Am=C3=A9rico Wang a= =C3=A9crit : >>>> 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=A9= crit : >>>> >> >>>> >> Hi >>>> >> >>>> >> CC netdev, since you ask questions about network stuff _and_ rw= lock >>>> >> >>>> >> >>>> >>> I'm using TILEPro and its rwlock in kernel is a liitle differe= nt than >>>> >>> other platforms. It have a priority for write lock that when t= ried it >>>> >>> will block the following read lock even if read lock is hold b= y >>>> >>> 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 wit= h lkml >>>> >> and Linus himself if I remember well. >>>> >> >>>> >It seems not a problem that read_lock() can be nested or not sinc= e >>>> >rwlock doesn't have 'owner', it's just that should we give >>>> >write_lock() a priority than read_lock() since if there have a lo= t >>>> >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-bia= s, >> 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. :) Note, on Tile, it uses a little different algorithm.