linux-kernel - Re: [PATCH net 1/5] net: netdevsim: fix inconsistent carrier state after link/unlink

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Message-ID: <e8180dc5-fc23-4044-bd67-92fc3eebdaa0@lunn.ch>
Date: Mon, 29 Dec 2025 19:39:29 +0100
From: Andrew Lunn <andrew@...n.ch>
To: yk@...oj.net
Cc: Jakub Kicinski <kuba@...nel.org>, Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	Breno Leitao <leitao@...ian.org>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net 1/5] net: netdevsim: fix inconsistent carrier state
 after link/unlink

On Tue, Dec 30, 2025 at 03:32:34AM +0900, yk@...oj.net wrote:
> From: Yohei Kojima <yk@...oj.net>
> 
> This patch fixes the edge case behavior on ifup/ifdown and
> linking/unlinking two netdevsim interfaces:
> 
> 1. unlink two interfaces netdevsim1 and netdevsim2
> 2. ifdown netdevsim1
> 3. ifup netdevsim1
> 4. link two interfaces netdevsim1 and netdevsim2
> 5. (Now two interfaces are linked in terms of netdevsim peer, but
>     carrier state of the two interfaces remains DOWN.)
> 
> This inconsistent behavior is caused by the current implementation,
> which only cares about the "link, then ifup" order, not "ifup, then
> link" order. This patch fixes the inconsistency by calling
> netif_carrier_on() when two netdevsim interfaces are linked.
> 
> This patch solves buggy behavior on NetworkManager-based systems which
> causes the netdevsim test to fail with the following error:
> 
>   # timeout set to 600
>   # selftests: drivers/net/netdevsim: peer.sh
>   # 2025/12/25 00:54:03 socat[9115] W address is opened in read-write mode but only supports read-only
>   # 2025/12/25 00:56:17 socat[9115] W connect(7, AF=2 192.168.1.1:1234, 16): Connection timed out
>   # 2025/12/25 00:56:17 socat[9115] E TCP:192.168.1.1:1234: Connection timed out
>   # expected 3 bytes, got 0
>   # 2025/12/25 00:56:17 socat[9109] W exiting on signal 15
>   not ok 13 selftests: drivers/net/netdevsim: peer.sh # exit=1
> 
> This patch also fixes timeout on TCP Fast Open (TFO) test because the
> test also depends on netdevsim.
> 
> Fixes: 1a8fed52f7be ("netdevsim: set the carrier when the device goes up")

Stable rules say:

   It must either fix a real bug that bothers people or just add a device ID.

netdevsim is not a real device. Do its bugs actually bother people?
Should this patch have a Fixes: tag?

       Andrew