From mboxrd@z Thu Jan 1 00:00:00 1970 From: =?utf-8?Q?Am=C3=A9rico?= Wang Subject: Re: Kernel rwlock design, Multicore and IGMP Date: Fri, 12 Nov 2010 16:19:45 +0800 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-Transfer-Encoding: QUOTED-PRINTABLE Cc: =?utf-8?Q?Am=C3=A9rico?= Wang , Cypher Wu , linux-kernel@vger.kernel.org, netdev To: Eric Dumazet Return-path: Content-Disposition: inline In-Reply-To: <1289546874.17691.1774.camel@edumazet-laptop> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Fri, Nov 12, 2010 at 08:27:54AM +0100, Eric Dumazet wrote: >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=A9c= rit : >> >> >> >> Hi >> >> >> >> CC netdev, since you ask questions about network stuff _and_ rwlo= ck >> >> >> >> >> >>> 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 tri= ed 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 ip= tables >> >> (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. >>=20 > >AFAIK, Lockdep allows read_lock() to be nested. > >> It is a known weakness of rwlock, it is designed like that. :) >>=20 > >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.