Messages in this thread | | | Date | Fri, 12 Nov 2010 19:06:47 +0800 | Subject | Re: Kernel rwlock design, Multicore and IGMP | From | Cypher Wu <> |
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2010/11/12 Américo Wang <xiyou.wangcong@gmail.com>: > 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@gmail.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@gmail.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. :) > > Note, on Tile, it uses a little different algorithm. >
It seems that rwlock on x86 and tile have different behavior, x86 use RW_LOCK_BIAS, when read_lock() it will test if the lock is 0, and if so then the read_lock() have to 'spinning', otherwise it dec the lock; when write_lock() tried it first check if lock is It seems that rwlock on x86 and tile have different behavior, x86 use RW_LOCK_BIAS and if so, set lock to 0 and continue, otherwise it will 'spinning'. I'm not very familiar with x86 architecture, but the code seems like working that way. -- 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/
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