netdev - Re: [RFC net-next 2/2] udp: No longer use SLAB_DESTROY_BY

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Date:	Mon, 28 Mar 2016 13:01:33 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Rick Jones <rick.jones2@....com>
Cc:	Eric Dumazet <edumazet@...gle.com>,
	"David S . Miller" <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Tom Herbert <tom@...bertland.com>
Subject: Re: [RFC net-next 2/2] udp: No longer use SLAB_DESTROY_BY_RCU

On Mon, 2016-03-28 at 12:11 -0700, Rick Jones wrote:

> I was under the impression that individual DNS queries were supposed to 
> have not only random DNS query IDs but also originate from random UDP 
> source ports.  https://tools.ietf.org/html/rfc5452 4.5 at least touches 
> on the topic but I don't see it making it hard and fast.  By section 10 
> though it is more explicit:
> 
>     This document recommends the use of UDP source port number
>     randomization to extend the effective DNS transaction ID beyond the
>     available 16 bits.
> 
> That being the case, if indeed there were to be 30000-odd concurrent 
> requests outstanding "upstream" from that location there'd have to be 
> 30000 ephemeral ports in play.

Sure, so these heavy duty programs have to consider the randomness
implications already, when picking a port in their pool.

Using the kernel and force socket()/bind()/close() in the hope to get
random values is safe, but never had been optimal for performance.

Note : file structures got RCU freeing back in 2.6.14, and I do not
think named users ever complained about added cost ;)