Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756118Ab0KLJJv (ORCPT ); Fri, 12 Nov 2010 04:09:51 -0500 Received: from mail-qy0-f181.google.com ([209.85.216.181]:51220 "EHLO mail-qy0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755413Ab0KLJJs convert rfc822-to-8bit (ORCPT ); Fri, 12 Nov 2010 04:09:48 -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=L8qCmgj/MbDvK5e/WEif15iwgY+enca1bYD2wTV1+vINu9gsPhzj/1qXUqJjChNpc9 DvxrRI/FK0kCPKovT/f8vENYdmphZY4ayZ7ZylV/ycXf7z4LKDsrJc/swr5pTENVZUxW CNXDd8ZMJHZEqfD7hFG0dLUqrLMxUjIY3vKOY= MIME-Version: 1.0 In-Reply-To: <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> <20101112081945.GA5949@cr0.nay.redhat.com> Date: Fri, 12 Nov 2010 17:09:45 +0800 Message-ID: Subject: Re: Kernel rwlock design, Multicore and IGMP From: Yong Zhang To: =?UTF-8?Q?Am=C3=A9rico_Wang?= Cc: Eric Dumazet , Cypher Wu , linux-kernel@vger.kernel.org, netdev Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2797 Lines: 74 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. Thanks, Yong > > >>> 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/ > -- 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/