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    SubjectRe: Kernel rwlock design, Multicore and IGMP
    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@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. :)

    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). :)
    Or am I missing something?

    Thanks,
    Yong

    >
    > Note, on Tile, it uses a little different algorithm.
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