Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755131Ab0KLIPZ (ORCPT ); Fri, 12 Nov 2010 03:15:25 -0500 Received: from mail-qy0-f174.google.com ([209.85.216.174]:37380 "EHLO mail-qy0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752052Ab0KLIPX (ORCPT ); Fri, 12 Nov 2010 03:15:23 -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=iRN9Rfnt+wJ5bhrwF0YmEs2WSjRXTCuUBcgfDGo+uLOAPT6V3WuP88ufv4d+AdPHM1 KC1b97ATMDLkVMRhyLjrEiltLo3WpyI2Fz7XjS+EObnIdAF9l/PWI7BO2oArSzFhkupp ipGA+xxWM3NxJuyb6gCxqfJl1Y5uZZnj+kNSo= Date: Fri, 12 Nov 2010 16:19:45 +0800 From: =?utf-8?Q?Am=C3=A9rico?= Wang To: Eric Dumazet Cc: =?utf-8?Q?Am=C3=A9rico?= Wang , Cypher Wu , linux-kernel@vger.kernel.org, netdev Subject: Re: Kernel rwlock design, Multicore and IGMP Message-ID: <20101112081945.GA5949@cr0.nay.redhat.com> References: <1289489007.17691.1310.camel@edumazet-laptop> <20101112071323.GB5660@cr0.nay.redhat.com> <1289546874.17691.1774.camel@edumazet-laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1289546874.17691.1774.camel@edumazet-laptop> 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: 2311 Lines: 60 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. >> 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. > >In the IGMP case, it should be easy for the task owning a read_lock() to >pass a parameter to the called function saying 'I already own the >read_lock(), dont try to re-acquire it' > >A RCU conversion is far more complex. > Yup. -- 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/