Re: is synchronize_net in inet_register_protosw necessary?

[junjie cai <junjiec@gmail.com>]

Hello,Paul,

Thank you very much for your reply.
So may I think that it would be ok to remove the synchronize_net 
from  inet_register_protosw on a UP platform?
I am doing embeded development so I don't have SMP to do the test.
I think it is ok for me to do just local modify to fit our needs.
Thanks again.
                                 junjie

On 6/17/05, Paul E. McKenney <paulmck@us.ibm.com> wrote:
> 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
>