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Message-ID: <20101113062824.GC3837@hack> Date: Sat, 13 Nov 2010 14:28:24 +0800 From: Américo Wang <xiyou.wangcong@...il.com> To: Yong Zhang <yong.zhang0@...il.com> Cc: Américo Wang <xiyou.wangcong@...il.com>, Eric Dumazet <eric.dumazet@...il.com>, Cypher Wu <cypher.w@...il.com>, linux-kernel@...r.kernel.org, netdev <netdev@...r.kernel.org> Subject: Re: Kernel rwlock design, Multicore and IGMP On Fri, Nov 12, 2010 at 09:00:17PM +0800, Yong Zhang wrote: >On Fri, Nov 12, 2010 at 05:18:18PM +0800, Américo Wang wrote: >> On Fri, Nov 12, 2010 at 05:09:45PM +0800, Yong Zhang wrote: >> >On Fri, Nov 12, 2010 at 4:19 PM, Américo Wang <xiyou.wangcong@...il.com> 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 <eric.dumazet@...il.com> 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. >> > >> >> Try >> >> % grep RW_LOCK_BIAS -nr arch/x86 >> >> *And* read the code to see how it works. :) > >If read_lock()/write_lock() fails, the subtracted value(1 for >read_lock() and RW_LOCK_BIAS for write_lock()) is added back. >So reader and writer will contend on the same lock fairly. > >And RW_LOCK_BIAS based rwlock is a variant of sighed-test >rwlock, so it works in the same way to highest-bit-set mode >rwlock. > >Seem you're cheated by it's name(RW_LOCK_BIAS). :) Ah, no, I made a mistake that I thought the initial value of rwlock is something like 0, but clearly it is RW_LOCK_BIAS. Yeah, then there is certainly no bias to writers, and x86 must be using almost the same algorithm with Tile. -- Live like a child, think like the god. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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