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From: Florian Westphal <fw@strlen.de>
Subject: Re: [PATCH 3/3] netfilter: ctnetlink: allow userspace to set labels
Date: Fri, 30 Nov 2012 15:02:54 +0100
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Cc: Florian Westphal <fw@strlen.de>,
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To: Pablo Neira Ayuso <pablo@netfilter.org>
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Pablo Neira Ayuso <pablo@netfilter.org> wrote:
> Quick idea: I think we can also support atomic replacement at word
> size using xchg, so the replacement operation can still happen at word
> level. Setting many bits at once would be also faster with that.

Unfortunately, no -- the interface is too rudimentary.
Example: You want to set bis 0, 2, 6; but leave all other
bit that are set intact.

So you first need to make a dump to fetch the current labels set.
Then, you set bits 0, 2, 6 and send the new state to the kernel.

But between the dump and the set operation, a new bit might
have been set.  So even when using xchg it will be un-set again...