netdev - Re: [PATCH net-next 1/2] udp: Simplify __udp*_lib_mcast

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Date:	Wed, 16 Jul 2014 08:58:02 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Held <drheld@...gle.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next 1/2] udp: Simplify __udp*_lib_mcast_deliver.

On Tue, 2014-07-15 at 21:53 -0400, David Held wrote:
> On Tue, Jul 15, 2014 at 8:14 PM, David Miller <davem@...emloft.net> wrote:
> >
> >
> > I think you are changing the logic of the loop in the edge case.
> >
> > If we have exactly ARRAY_SIZE(stack) sockets to process, the old code
> > performs the flush_stack() outside of the hslot->lock, but with your
> > change we'll do it inside the lock.
> >
> > The tradeoff here is reducing hslot->lock hold times vs. avoiding
> > taking a hold on all the sockets in the stack[] array.
> >
> > I just wanted to point this out and make sure you are aware of and
> > are ok with this.
> 
> The followup patch makes it so we always take a hold on the sockets
> anyway (it makes things simpler and only changes things for the too
> large array case), so might as well reduce the lock time for the
> exactly ARRAY_SIZE case.

Note that my initial motivation for this array was to convert the
multicast lookup into a RCU one, but I never finished this.

If we hold a spinlock, there is no need for this array in the first
place.

If we dont hole a spinlock, we are forced to temporarily store the
sockets into the array, just in case we restart the lookup because we
are directed into another chain.



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