Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756080Ab0KLJNk (ORCPT ); Fri, 12 Nov 2010 04:13:40 -0500 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 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=PyxucQi2MZuVnTolb8dtL2M79h8YsmpEHIogh/BGo1Sk6KaHBtQBBaP94B3AJXIyWt UwgBnpZU7X/ko21/DjZouhfPHDcmfvHbH057TyPg/IpX+F5hgFXPVeuhPWm0dwUdKwVI Fg/tZ5G97tIgIfTUnUXPR7fkN1A9H5t89ulWg= Date: Fri, 12 Nov 2010 17:18:18 +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: <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-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2394 Lines: 62 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. :) Note, on Tile, it uses a little different algorithm. -- 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/