Re: Kernel rwlock design, Multicore and IGMP

[Yong Zhang]

On Fri, Nov 12, 2010 at 4:19 PM, AmÃrico Wang <xiyou.wangcong@xxxxxxxxx> wrote:
> On Fri, Nov 12, 2010 at 08:27:54AM +0100, Eric Dumazet wrote:
>>Le vendredi 12 novembre 2010 Ã 15:13 +0800, AmÃrico Wang a Ãcrit :
>>> 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@xxxxxxxxx> wrote:
>>> >> Le jeudi 11 novembre 2010 Ã 21:49 +0800, Cypher Wu a Ãcrit :
>>> >>
>>> >> 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.

Thanks,
Yong
>
>
>>> 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.
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