netdev - Re: [PATCH] net: do not allow changing SO_REUSEADDR/SO

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Date:   Mon, 11 Jun 2018 15:29:57 -0700
From:   Maciej Żenczykowski <zenczykowski@...il.com>
To:     Marc Dionne <marc.c.dionne@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        netdev <netdev@...r.kernel.org>,
        Jeffrey Altman <jaltman@...istor.com>,
        Marc Dionne <marc@...istor.com>
Subject: Re: [PATCH] net: do not allow changing SO_REUSEADDR/SO_REUSEPORT on
 bound sockets

> This change is a potential performance regression for us.  Our
> networking code sets up multiple sockets used by multiple threads to
> listen on the same udp port.  For the first socket, the bind is done
> before setting SO_REUSEPORT so that we can get an error and detect
> that the port is currently in use by a different process, possibly a
> previous incarnation of the same application.
>
> With this change, the servers end up with a single listener socket and
> thread, which can have a large impact on performance for a busy
> server.
>
> While we can adjust for the new behaviour going forward, this could be
> an unpleasant surprise for our customers who get this update when
> moving to a new kernel version or through a backport to stable
> kernels, if this is targeted for stable.

Probably you can:
fd1=socket()
fd2=socket()
bind(fd1,port=0)
setsockopt(fd2,REUSEPORT,1)
port = getsockname(fd1)
close(fd1)
bind(fd2,port)

Although yeah there's a slight chance of a race (ie. 2nd bind failing,
in which case close() and retry).