Re: [PATCH net-next-2.6] net: add rcu protection to netdev->ifalias

[Stephen Hemminger <shemminger@vyatta.com>]

On Wed, 18 May 2011 00:18:12 +0200
Eric Dumazet <eric.dumazet@gmail.com> wrote:

> Le lundi 02 mai 2011 à 15:27 -0700, David Miller a écrit :
> > From: Stephen Hemminger <shemminger@vyatta.com>
> > Date: Thu, 28 Apr 2011 08:43:37 -0700
> > 
> > > On Thu, 28 Apr 2011 10:56:07 +0200
> > > Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > > 
> > >> Four years ago, Patrick made a change to hold rtnl mutex during netlink
> > >> dump callbacks.
> > >> 
> > >> I believe it was a wrong move. This slows down concurrent dumps, making
> > >> good old /proc/net/ files faster than rtnetlink in some situations.
> > >> 
> > >> This occurred to me because one "ip link show dev ..." was _very_ slow
> > >> on a workload adding/removing network devices in background.
> > >> 
> > >> All dump callbacks are able to use RCU locking now, so this patch does
> > >> roughly a revert of commits :
> > >> 
> > >> 1c2d670f366 : [RTNETLINK]: Hold rtnl_mutex during netlink dump callbacks
> > >> 6313c1e0992 : [RTNETLINK]: Remove unnecessary locking in dump callbacks
> > >> 
> > >> This let writers fight for rtnl mutex and readers going full speed.
> > >> 
> > >> It also takes care of phonet : phonet_route_get() is now called from rcu
> > >> read section. I renamed it to phonet_route_get_rcu()
> > >> 
> > >> Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
> > >> Cc: Patrick McHardy <kaber@trash.net>
> > >> Cc: Remi Denis-Courmont <remi.denis-courmont@nokia.com>
> > > 
> > > Acked-by: Stephen Hemminger <shemminger@vyatta.com>
> > 
> > Applied, thanks everyone!
> 
> I think we probably have to make some fixes, because rtnl_fill_ifinfo()
> can access fields that were previously protected with RTNL
> 
> Let's see if it's doable, before considering re-adding rtnl_lock() in
> rtnl_dump_ifinfo() ?
> 
> First step : dev->ifalias
> 
> Its late here, I'll continue the work tomorrow.
> 
> Thanks
> 
> [PATCH net-next-2.6] net: add rcu protection to netdev->ifalias
> 
> Avoid holding RTNL in show_ifalias() and rtnl_fill_ifinfo() to
> manipulate netdev->ifalias
> 
> Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>

I am beginning to think the idea of optimizing case of "ip link show"
running in background with adding/removing network devices is silly. 
And switching to RCU leads to all sorts of corner cases where only
partial data will be read. Is it really worth it?