From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eric Dumazet Subject: Re: Kernel rwlock design, Multicore and IGMP Date: Fri, 12 Nov 2010 08:27:54 +0100 Message-ID: <1289546874.17691.1774.camel@edumazet-laptop> References: <1289489007.17691.1310.camel@edumazet-laptop> <20101112071323.GB5660@cr0.nay.redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Cypher Wu , linux-kernel@vger.kernel.org, netdev To: =?ISO-8859-1?Q?Am=E9rico?= Wang Return-path: 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 In-Reply-To: <20101112071323.GB5660@cr0.nay.redhat.com> Sender: netdev-owner@vger.kernel.org List-ID: Le vendredi 12 novembre 2010 =C3=A0 15:13 +0800, Am=C3=A9rico Wang a =C3= =A9crit : > 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 =C3=A0 21:49 +0800, Cypher Wu a =C3=A9cr= it : > >> > >> Hi > >> > >> CC netdev, since you ask questions about network stuff _and_ rwloc= k > >> > >> > >>> 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 trie= d 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 ipt= ables > >> (it can re-enter itself), > >> and we used instead a spinlock(), but with many discussions with l= kml > >> 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. >=20 AFAIK, Lockdep allows read_lock() to be nested. > It is a known weakness of rwlock, it is designed like that. :) >=20 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() t= o 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.