netdev - Re: [PATCH net-next] tc: fix tc actions in case of shared skb

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Message-ID: <55A425C4.60301@iogearbox.net>
Date:	Mon, 13 Jul 2015 22:55:32 +0200
From:	Daniel Borkmann <daniel@...earbox.net>
To:	Alexei Starovoitov <ast@...mgrid.com>
CC:	David Miller <davem@...emloft.net>, jhs@...atatu.com,
	jiri@...nulli.us, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] tc: fix tc actions in case of shared skb

On 07/13/2015 10:17 PM, Alexei Starovoitov wrote:
...
> We cannot check tc actions from pktgen, since they can be added
> dynamically.
> So I see three options:
> 1 get rid of burst hack for both RX and TX in pktgen (kills performance)
> 2 add unlikely(skb_shread) check to few tc actions
> 3 do nothing
>
> I think 2 isn't that bad after all if properly documented with
> "because pktgen is doing this hack for performance" ?
>
> I'm fine with 3 too, since the whole pktgen business is for root
> and for kernel hackers who suppose to know what they're doing.

Hmm, one thing for option 3 could be that we add a modinfo tag
"experimental", so that on loading of pktgen module, we trigger
(like in case of staging) ...

   add_taint_module(mod, TAINT_CRAP, LOCKDEP_STILL_OK);

... and add a pr_warn() to the user, it may be more visible/clear
than the "Packet Generator (USE WITH CAUTION)" Kconfig title? ;)

It'd be a pity that we'd need the extra atomic read only for the
pktgen case. :/ With regards to option 2, you could hide that behind
a static inline helper wrapped in IS_ENABLED(CONFIG_NET_PKTGEN), but
that is a veeeery ugly workaround/hack as well (and distros might
even ship it nevertheless). I wouldn't be surprised if there are
other usage combinations with pktgen that would crash your box. :/

Best,
Daniel
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