netdev - Re: [PATCH 1/1] net: Add SO_REUSEPORT_LISTEN

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Message-ID: <20151218185812.GD4448@1wt.eu>
Date:	Fri, 18 Dec 2015 19:58:12 +0100
From:	Willy Tarreau <w@....eu>
To:	Josh Snyder <josh@...e406.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Tom Herbert <tom@...bertland.com>,
	Tolga Ceylan <tolga.ceylan@...il.com>,
	Aaron Conole <aconole@...heb.org>,
	"David S. Miller" <davem@...emloft.net>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH 1/1] net: Add SO_REUSEPORT_LISTEN_OFF socket option as drain mode

Hi Josh,

On Fri, Dec 18, 2015 at 08:33:45AM -0800, Josh Snyder wrote:
> I was also puzzled that binding succeeded. Looking into the code paths
> involved, in inet_csk_get_port, we quickly goto have_snum. From there, we end
> up dropping into tb_found. Since !hlist_empty(&tb->owners), we end up checking
> that (tb->fastreuseport > 0 && sk->sk_reuseport && uid_eq(tb->fastuid, uid)).
> This test passes, so we goto success and bind.
> 
> Crucially, we are checking the fastreuseport field on the inet_bind_bucket, and
> not the sk_reuseport variable on the other sockets in the bucket. Since this
> bit is set based on sk_reuseport at the time the first socket binds (see
> tb_not_found), I can see no reason why sockets need to keep SO_REUSEPORT set
> beyond initial binding.
> 
> Given this, I believe Willy's patch elegantly solves the problem at hand.

Great, thanks for your in-depth explanation.

Eric, do you think that this patch may be acceptable material for next
merge window (given that it's not a fix per-se) ? If so I'll resubmit
later.

Thanks,
Willy

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