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    SubjectRe: is synchronize_net in inet_register_protosw necessary?
    On Thu, Jun 16, 2005 at 10:15:02AM -0700, Paul E. McKenney wrote:
    > On Tue, Jun 14, 2005 at 11:18:08PM +0900, junjie cai wrote:
    > > hi all.
    > > i am a newbie to linux kernel.
    > > in a arm926 board i found that it took about 30ms to finish
    > > the (net/ipv4/af_inet.c:898) inet_register_protosw
    > > because of the synchronize_net call during profiling.
    > > synchronize_net finally calls synchronize_rcu, so i think
    > > this is to make the change visiable after a list_add_rcu.
    > > but according to the Document/listRCU.txt it seems that
    > > a list insertation does not necessarily do call_rcu etc.
    > > may i have any mistakes, please kindly tell me.
    > From a strict RCU viewpoint, you are quite correct. But sometimes
    > the overall locking protocol (which almost always includes other things
    > besides just RCU) places additional constraints on the code. My guess is
    > that the networking folks needed to ensure that the new protocol is seen
    > by all packets that are received after inet_register_protosw() returns.
    > But I need to defer to networking guys on this one.

    Hello, Junjie,

    Ran into one of the networking guys off-list. Apparently, the
    synchronize_net() is there out of paranoia. It might be necessary,
    but he could not think of a reason for its being there. If you want
    to shave 30ms off of your boot time by removing it, here is his
    suggested test procedure:

    o Write a small dummy protocol as a module.

    o On an SMP machine, have one process repeated modprobe/rmmod
    while another process repeatedly does socket() calls for
    the dummy protocol.

    Thanx, Paul
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