Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753974Ab0KLH2A (ORCPT ); Fri, 12 Nov 2010 02:28:00 -0500 Received: from mail-ww0-f44.google.com ([74.125.82.44]:60562 "EHLO mail-ww0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751837Ab0KLH16 (ORCPT ); Fri, 12 Nov 2010 02:27:58 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=subject:from:to:cc:in-reply-to:references:content-type:date :message-id:mime-version:x-mailer:content-transfer-encoding; b=w2D+A88Kag5bM7YBkvH8z7bfUPK/KMQodZnE1HbRh4SdsIB4ArXcTSZi+xF8/qB2OK kYO8n7T/tkXolS0VaBMASTuk14VG+8QjTlG79OsUyYFmXnzUTqGzu0y2KsF4jldDA1It ve6Uq/jNf8/tBhLgXnjrLCRhHU5f6reoILGU0= Subject: Re: Kernel rwlock design, Multicore and IGMP From: Eric Dumazet To: =?ISO-8859-1?Q?Am=E9rico?= Wang Cc: Cypher Wu , linux-kernel@vger.kernel.org, netdev In-Reply-To: <20101112071323.GB5660@cr0.nay.redhat.com> References: <1289489007.17691.1310.camel@edumazet-laptop> <20101112071323.GB5660@cr0.nay.redhat.com> Content-Type: text/plain; charset="UTF-8" Date: Fri, 12 Nov 2010 08:27:54 +0100 Message-ID: <1289546874.17691.1774.camel@edumazet-laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.30.3 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2072 Lines: 54 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. > 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. -- 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/