linux-kernel - Re: [PATCH net v2 1/2] net: netdevsim: fix inconsistent carrier state after link/unlink

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Message-ID: <4xxshlplnoulncmfkinnuzvwx7bbnukkwds7go75cese4scrws@6xlfsy35t7gg>
Date: Wed, 31 Dec 2025 02:16:53 -0800
From: Breno Leitao <leitao@...ian.org>
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>, Andrew Lunn <andrew@...n.ch>, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net v2 1/2] net: netdevsim: fix inconsistent carrier
 state after link/unlink

Hello Yohei,

On Wed, Dec 31, 2025 at 01:03:29AM +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.)

That seems a real issue, in fact. The carriers are only getting up when
opening the device, not when linking. Thus, this patch makes sense to
me.

> 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 fixes 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 solves timeout on TCP Fast Open (TFO) test in
> NetworkManager-based systems because it also depends on netdevsim's
> carrier consistency.
> 
> Fixes: 1a8fed52f7be ("netdevsim: set the carrier when the device goes up")
> Signed-off-by: Yohei Kojima <yk@...oj.net>

Reviewed-by: Breno Leitao <leitao@...ian.org>

Thanks for the fix!