linux-kernel - Re: [PATCH 4.19 10/79] udp: correct reuseport selection with connected sockets

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Date:   Sat, 21 Sep 2019 22:32:37 +0200
From:   Pavel Machek <pavel@...x.de>
To:     pavel@...x.de
Cc:     linux-kernel@...r.kernel.org,
        Willem de Bruijn <willemb@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Craig Gallek <kraig@...gle.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH 4.19 10/79] udp: correct reuseport selection with
 connected sockets


On Fri 2019-09-20 00:02:55, Greg Kroah-Hartman wrote:
> From: Willem de Bruijn <willemb@...gle.com>
> 
> [ Upstream commit acdcecc61285faed359f1a3568c32089cc3a8329 ]
> 
> UDP reuseport groups can hold a mix unconnected and connected sockets.
> Ensure that connections only receive all traffic to their 4-tuple.
> 
> Fast reuseport returns on the first reuseport match on the assumption
> that all matches are equal. Only if connections are present, return to
> the previous behavior of scoring all sockets.
> 
> Record if connections are present and if so (1) treat such connected
> sockets as an independent match from the group, (2) only return
> 2-tuple matches from reuseport and (3) do not return on the first
> 2-tuple reuseport match to allow for a higher scoring match later.
> 
> New field has_conns is set without locks. No other fields in the
> bitmap are modified at runtime and the field is only ever set
> unconditionally, so an RMW cannot miss a change.

That's an ... extremely tricky game with concurrent access. I'm pretty
sure it is not valid C, but maybe it is acceptable for kernel.

> --- a/include/net/sock_reuseport.h
> +++ b/include/net/sock_reuseport.h
> @@ -21,7 +21,8 @@ struct sock_reuseport {
>  	unsigned int		synq_overflow_ts;
>  	/* ID stays the same even after the size of socks[] grows. */
>  	unsigned int		reuseport_id;
> -	bool			bind_inany;
> +	unsigned int		bind_inany:1;
> +	unsigned int		has_conns:1;

But should it at least be commented here? If someone adds another int :1,
he may get a surprise...

Best regards,
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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