Re: Kernel rwlock design, Multicore and IGMP

["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.