Messages in this thread | | | Date | Fri, 12 Nov 2010 16:19:45 +0800 | From | Américo Wang <> | Subject | Re: Kernel rwlock design, Multicore and IGMP |
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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.
>> 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. >
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