Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757408Ab0KLLLB (ORCPT ); Fri, 12 Nov 2010 06:11:01 -0500 Received: from mail-qy0-f181.google.com ([209.85.216.181]:62252 "EHLO mail-qy0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757002Ab0KLLLA convert rfc822-to-8bit (ORCPT ); Fri, 12 Nov 2010 06:11:00 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type:content-transfer-encoding; b=Du/nmMXWmv16E7H4RyGF1YVgq6nPJVjvVBjpKcFXa7OixkBD2lIDhNTwGOUx+momdg MjLL3jm1iKwRToISAcQRQp9tsJe+ETW1AUTxdpD66efvqmJPRnOYV25fopPmW0B/b0WI KpEDUyZaEGrPEVoBJaBftPXK6HIuYoZxTEiB0= MIME-Version: 1.0 In-Reply-To: <1289546874.17691.1774.camel@edumazet-laptop> References: <1289489007.17691.1310.camel@edumazet-laptop> <20101112071323.GB5660@cr0.nay.redhat.com> <1289546874.17691.1774.camel@edumazet-laptop> Date: Fri, 12 Nov 2010 19:10:58 +0800 Message-ID: Subject: Re: Kernel rwlock design, Multicore and IGMP From: Cypher Wu To: Eric Dumazet Cc: =?ISO-8859-1?Q?Am=E9rico_Wang?= , linux-kernel@vger.kernel.org, netdev Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2464 Lines: 62 On Fri, Nov 12, 2010 at 3:27 PM, Eric Dumazet wrote: > Le vendredi 12 novembre 2010 ? 15:13 +0800, Am?rico Wang a ?crit : >> 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 ? 21:49 +0800, Cypher Wu a ?crit : >> >> >> >> 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. > >> 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' I used to using that way, just seperate the call internal and external, with external one hold lock then call the internal one. But in that case ip_check_mc() is called indirectly from igmpv3_sendpack() and is not very clear how to give the different paramter? > > A RCU conversion is far more complex. > > > > -- 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/